I agree that Facebook can sometimes be a time-sucking waste, but I can think of things it does well that no other service does:
* It keeps me connected to people who are far away. I grew up estranged from the rest of the family. I'm friends with a dozen cousins on Facebook that I haven't seen since I was four years old.
* It helps me organize creative projects. I do a lot of collaborative work with other artists, and throwing everyone into a Facebook group where we can communicate, post photos, and share links is so much easier than using just about any other platform.
I try to jump on, do what needs to be done, and leave whenever possible.