For a lot of social circles, facebook is the only service that almost everyone will regularly check and that lets you coordinate events. My friends who don't use facebook will regularly miss (or not get invited to) parties, book club, skiing trips, etc. This isn't because no one likes them or thinks to invite them, it's because they opted out of the platform.
The other big point of stickiness that I can think of is that facebook is many people's cloud. Years of photos are backed up there and only there. It's unfortunate that they've relinquished control of their files to a closed-platform third party, but that's the way it is.