Maybe I just have friends who don't care too much, but I wouldn't expect any of my friends to notify me if they're going to a run of the mill event without me, but they would suggest it/notify on Facebook about it. It's a lot less personal, I feel, to notify someone on Facebook than to message someone personally and invite them. Chances are, they'd see I'm no longer on Facebook and think I've gone into some kind of social seclusion.

It also doesn't help solve the problem of me finding something I want to go to (that my friends might be interested in) but they didn't see themselves. There was recently a Studio Ghibli film showing in my city, and after I found it on FB, I suggested it to two friends and we went. I wouldn't have found that event without FB, and even if I did, it would feel awkward (for an equally introverted person like me) to specifically message the two people and ask - now they have some kind of obligation, and they can't claim they just haven't checked Facebook in a while if I sent them a text/Telegram/whatever.