I find it odd that people say things like "that's what we have email and messenger and calendars and sms for". That's the point - I've replaced 5 programs with 1. I still use email and sms (messenger, not so much). But it's now all in one place, with all the people I care about right there, without having to start exchanging usernames.
Sure I'd jump ship if another platform provided more value, but that's (somewhat) different then a fad.
funny enough, i never set facebook status updates or read anyone else's. i guess creating an event posts it in your stream, but the direct-invite and invited-by-friends feature is the extent of my use of facebook (people do message me on there and i reply via email).
my parties generally have a good mix of highschoolers to 60 somethings. i'll grant you that 2/3 of the sixty somethings are my neighbors and they get the email invite, but a lot of the 50-plus social crowd in portland, OR is on facebook.
the other benefit to using a tool like facebook to do (open) party invites is that people can invite their friends. most of my parties are open; i trust my friends' judgment not to bring/invite assholes. and it works out remarkably well. i've hosted thousands of people at around a hundred parties (some are smaller dinner parties, but still open invite) since i started this strategy, at least half of whom i did not know at all, and only 2 things have ever been broken/stolen. total value lost: $80. total connectedness gained & fun had? incalculable (but huge).
WORKSFORME, without being my only or even main method of socialization (which is decidedly in real life).
Facebook may go away, but the reasons why people use sites like Facebook will not. And it's really important to try and discover what those reasons are, so that you can offer that value in your own product.
Drugs are horrible for your body and for your life, but people still use it because it offers something they want. You can sit back and call drugs a fad, or you can look into why people use drugs. With that knowledge, you can invent something radically new that is healthy, and replaces drugs. Or, you can just invent cheaper, stronger drugs. Either way, you'll be rich.