1) It’s a time sink.
2) Your privacy - what you look like, who you associate with, what you do - is recorded, and that information belongs to a company with thousands of people trying to think of ways to commercialize that information. You probably don’t agree with everything they will do.
3) When you make an ass out of yourself (and we all do at some point) it goes to EVERYONE, not just one or two people. And it stays around forever.
And yet people still say that this wasn't the REAL Trayvon (at first they claimed photoshopping even).