Why I use Facebook (a story of web development)
The reason I actually use my Facebook account is because of my bike club. Explaining this reason reveals something sad about the state of the web and putting together web sites, and why Facebook is so attractive. I care about this because in some quarters, it's popular to be dismissive of people and groups who use Facebook despite the many problems with it, and my view is that people who have this attitude may not understand the good reasons that push people to Facebook.
(And in turn this matters because if we want to get people off Facebook, we have to understand why they're on there in the first place.)
I'd probably have a Facebook account no matter what, simply because a number of people I know use Facebook heavily and being on Facebook feels necessary to maintain connections to them. But without my bike club I probably wouldn't log on more than once every few months. What drives me to log on more often than that (at least during the biking part of the year) is that my bike club uses Facebook (in the form of a Facebook Group) as its de facto member discussion forum. Sure, our web site has a 'forum' section, but I assure you that's it's a pale shadow compared the Facebook Group. Especially, during bike riding season the Facebook Group is the place you want to look for last minute announcements about rides (whether cancellations forced by weather, route updates, or what have you).
So, why is a Facebook Group so attractive for this? The simple version of the answer is that running your own discussion forum is a pain in the rear and you don't get much for it. Facebook spends huge amounts of effort making what is in effect very good discussion forum software with a ton of advanced features, and if you use them not only can you take advantage of all of that work, you get account management for free (often using accounts that people already have). You don't have to sort through various discussion forum software to pick one, figure out if it can go on your web host, integrate it into your website, manage the configuration, worry about security issues, handle people who forget their account password or have it compromised, etc etc etc. Nor do you have to worry about storing photos (or even videos) that members may want to upload, or managing photo galleries, or a huge number of other features that Facebook gives you for free.
(One of those features is spam management, by the way, which is very important if your forum needs to be open to the public. The bike club's forum doesn't have to be (although it's handy if non-members can talk to us), but Facebook is also where various sections of Toronto's cycling activist community are active, and that has to be open to the public.)
If you're a computer savvy person doing things for a computer savvy audience, sure, maybe running your own discussion forum software is something you're willing to tackle. But we're talking about a bike club here. Our goal is not running a website, our goal is to go on bike rides; the website is sort of incidental to the process (although still crucial to it).
In short, the wonder is not that people turn to Facebook for discussion forums and similar things. The wonder is that not everyone does. Facebook is where people go to get together and to organize because it almost always beats the hell out of setting all of that up for yourself.
(I don't have any good ideas for how to avoid this, especially since all of the alternatives that sound plausibly attractive to organizations like my bike club would pretty much have to be central monoliths that you can also outsource the entire forum to, and it's far from clear that they'd be much better than Facebook. Running this sort of infrastructure is hard, especially once you take on the vital additional tasks of account management (which shouldn't just be delegated to Facebook) and anti-spam work.)
(I periodically mention a short version of this on Twitter, so today I felt like writing the long version up.)