To me, the end of facebook was when facebook started trying too hard to dictate to me how I would use the site.
I don't see a chronological timeline of what my friends are doing (a la twitter), I see some posts that facebook thinks I'll be interested in.
What the hell is that, facebook? Most of the things I see now are from people I really don't care about at all. The only reason I ever log in anymore is to see what a group I belong to on there is up to.
And what have you done to the pictures? Why can't I right click them and send a link to one of my friends? Facebook, maybe you've forgotten this part, but those annoying people using your website are the reason that your website exists at all. When you take stuff like that away, and when it's obvious why you're doing it (you don't want people to see facebook's pictures without facebook's branding on them [the container page, I mean, that you'll see if you use an approved sharing mechanism to share pictures]), it feels like a giant slap in the face.
There is nothing fun happening on facebook anymore, and the things that I've posted to it in the past are now impossible to find (keep scrolling and scolling and scrolling and triggering that javascript to load more things, right!?).
I don't really feel like I'm connected to the people that I'm supposedly "connected" to anymore. It sucks.
The other reason I pretty much never use facebook, is http://thingist.com/ -- a few people have said that it reminds them a lot of '05, '06 era facebook, which is cool to hear. There are no apps, there is no algo to determine what it shows you, it's just a very simple way of sharing things with a group of people (and, more importantly, keeping it organized into a list [songs I like, quotes, tools for programmers, documentaries you should watch, etc]).
Full disclosure: thingist is my side project, and my addiction. Obviously I'm not a designer, so the design is horrible, but the functionality works, and I'm addicted to it.