Online communication services can all be classified according to different aspects. A chat IM message is, as an example, usually a one-to-one realtime communication. There is an assumption that I will answer a chat message rather quickly - even when I don't have to. IRC - Internet relay chat - is different: communication is topical as I join a channel discussing a certain topic. Communication is many-to-many and there is no assumption I answer the message right away. Likewise for emails - but there exchange of messages happen at a slower pace with larger bodies of text. The reason you use more than one service is essentially that different communication services cater to different kinds of communication. The collaborative benefits you gain when developing source code over IRC is not paralleled in any place currently for instance - not as a free open project at least.
What kind of communication services disrupts your flow of thought by alerting you and requiring you to answer (phones, I am looking at you!)? How realtime/instant is a message? Can you post additional content like photos, images, urls? Can you split communication into topics of interest? Are topics invite-only or free-to-join by default? How is moderation done on topics? Is it easy to form groups of privacy to discuss something? Is the communication one-to-one-focused, or is it one-to-many or even perhaps many-to-many? Can you build robots to automate tasks for the service? And so, in the light of Google+:
Google+ major point is to work like Facebook does in the core. This means that I am not the primary customer - as I don't use Facebook much. Hence, I am probably the wrong guy to judge if it will work out or not. By the core of Facebook, I mean that the primary goals of G+ and FB overlap very much. In the long run, the goal is to form a relation-graph between people and get access to it for Google.
The major, essential, difference between Google+ and Facebook is the formation of connections. In Facebook, A adds B and then B confirms the relationship. The opposite of this is Twitter: you just follow people you are interested in. Facebook runs a 2-way handshake whereas Twitters is one-way. Since connections in Facebook is 2-way, then we trivially have: if A is connected to B, then B is also connected to A. In other words: the connection graph is undirected in Facebook. Contrast with Twitter: graphs in twitter are directed. There are numerous one-way connections in Twitter. Most notable are celebrity-figures with an extremely high amount of followers. The communication from such an account is essentially 1-to-many and public.
So what does Google+ do? It chooses a variant of Twitter. In G+, the graph is directed like in Twitter, but you also label the edges with a name -- your circle designation for the connection. You can have multiple edges to the same person/account, so in reality the graph is a multigraph where edges are discriminated by their circle-labels. Another representation is to deem that a graph edge is labeled with a set of circle-labels.
When A adds B to the circles, A is adding an edge in the graph from A to B for each circle. So A builds a one-way path to the other end. If later B decides to add A to his circles, then there will be edges in the opposite direction. They may not be labeled the same at all. This means that A can have B in the circle of close friends while B has A in the circle ObnoxiousCatPhotoSubmitters. This is an interesting skew of connections which come into play. When you post something to a circle, the recipients can not see the message as if it is originating from that circle - in fact circles are exclusive to an account and can't be seen by others. This in effect makes a message a low-level value which is exchanged from a sharer to a list of people the sharer decides via circles. Upon receipt, the message is then cast into the light of the recipients circles.
So forming connections is like in Twitter, only that edges are labeled by Circles given by the source. This is a rather genius move. When A shares, he chooses the circles he want to share the message with. The system then searches all edges labeled with those circles, and posts on those edges only. If A wants to post Tech-related stuff, A can do so in the Tech-circle so only people that might care is hit by the sharing.
- The sender, A, has complete control of who sees the message. This is what Google refers to as the ability of G+ to accommodate different kinds of privacy levels on messages. Also, it is the sender, A, that defines his or her own levels of privacy with Circles as they are local to an account. It also blurs the definition of "follower", "friend", "connection", "add" and so on. Some edges are labeled as "close friends" and are thus strong edges from a privacy perspective. Other edges are weak in the sense that you have a connection, but there are limits to what you want to share with the other person.
- When data arrives at B, messages from A gets sorted according to the circles of B. So when B looks at streams from circles in which A is in, he sees the shared information from A.
- Consequence: A needs to moderate what he sends along the Tech-circle to B. If A begins sending Cat photos to his tech-friends, he will end up in the ObnoxiousCatPosters circle at B - and only that circle. B is free to replace him at any time if he wants. The assumption currently is that relations between people are formed one primary ground. "He is family". "She and I are always discussing cool graphics design". This may largely be true.
- One thing I have not figured out yet: Suppose I want to follow B for more than one interest. Cats and Dogs. I have a circle of Cats and one of Dogs. But when B posts in his public circle for all to see, his posts will end up at my place in both Cats and Dogs. To the best of my knowledge there is no way I can currently split this. My guess is that Google will have to solve it in the longer run. They have several options among of which is to use statistical analysis on the posts through machine learning. Or allow #tags as in Twitter.
- Another thing I have not figured out completely yet: I don't think G+ can substitute for the other communication tools I am already using. Since I am not much of a Facebook user I am not the right demographic, but I don't see G+ replacing Email, IRC or Twitter. Twitter is the thing it will have the best options at replacing in my opinion. Email and IRC needs a more wave'esque approach to the problem I am afraid. I would really like to see an "IRC for the masses" and I am not convinced that Huddle is it. Further, the locality of circles to an account makes them distinctively different from IRC channels. On the other hand, if you have a topical connection graph, you can use machine learning to suggest IRC-like topics later on - and it is not that hard to add proper IRC-like group chat features to Google Chat which is already integrated.
So in conclusion: G+ is a funny hybrid between the style of Facebook and Twitter. The circle concept is central to how the connection-graph is formed. And thus, central to how G+ works. It will be interesting to see if Facebook will have a counter-solution. Currently, FB can create groups of friends and handle these in many ways like G+ can handle circles. We can only hope that this competition from Google will force Facebook to improve their system as well. But they probably can't get rid of the two-way connections easily.