I can't speak for other people, but the effort it took to maintain a Facebook presence (deidentify unflattering photos, take a few minutes every day to get caught up on people's minutiae) was far greater than the benefit it brought me.
People who want to talk to me know my email address and phone number.
(edit: clarification, first paragraph)
To me, it's analogous to the argument against text messaging. Yes, people who want to talk to me can simply call, but there's a bottom threshold of importance that must be met to deem a phone call necessary. Meanwhile I can text a one sentence message to a friend I haven't seen in months and not spend half an hour on the phone.
We could use Google calendar for this, or any number of other tools, but that would put a burden on our other friends to learn a new system. The end result would be that some people would flat out not attend the event, and some others would attend but not RSVP, throwing off guest counts, etc.
tl;dr I treat Facebook like a virus that is infecting the web.
I personally think FaceBook will reach an inflection point where public interest will wane and people will use it less and less... but still have accounts. Then something better will come along and there might be a mass migration. For now though, the reality is it's somewhat essential.