|(Second G should be lower-case.)|
I explained its purpose here last year, thusly:
The main purpose of this is to let physically or socially isolated atheists quickly and easily see where they might find some sympathetic ears, or at least to give them some hope that they aren't totally alone in the world. It can be hard to imagine if you've never felt that sort of isolation, but any little bit of contact with the somewhat-like minded can mean a lot.
A secondary purpose (as far as I'm concerned, anyway) is to put a dent in theists' assumptions about their dominance and prevalence. It's easy to assume that everyone is like you, especially when the ones who are different specifically reject going out of their way to look different and distinct. And that assumption can feed a terrible arrogance and lack of self-questioning. Shaking that up every now and then is good for everyone.
Both those things together add towards making public atheism more socially acceptable.
And then I had to write a piece grumbling about poor participation for A Week 2012. Harumph!
Considering how easy it is, and how much that primary purpose could be appreciated, it seems like a ridiculously cheap cost-to-benefit ratio. All you have to do is change your profile picture for a week; it could possibly be easier, but that's already got to be below the minimum difficulty threshold for even the laziest among us (which is why I'm able to participate). So, why not just fly an A? Isn't it more effort to think up a reasonable excuse not to, than to just damn well do it?
And think of the poor atheist children! There are atheists in the Third World who wish they had even one day's worth of lower-case a (some of them even have to be ithiests instead, because it's cheaper), so cherish your nice, big, shiny upper-case A for a whole week! You lucky alphabet gourmet.
I haven't been able to find a formal Google+ equivalent, but I see no reason to limit this to just one social media site. I'll be A'ing both my FB and G+ profiles between 17 and 23 March.