This, from Prosecutor’s Discretion, says it much more persuasively than I ever could:
Facebook, Twitter, Myspace (anyone use that anymore?), google+. I’m on record by saying I love them all. People need to brag about their crimes and what better way than in writing on Facebook. They love to threaten witnesses. They connect with a group of friends, which makes it easier to show their affiliation with a gang. They post pictures of themselves smoking weed, brandishing guns, or of their new tattoos. Plaintiffs in civil cases will post pictures dancing in bars, going for hikes, or lifting heavy objects, all activities they claim they are unable to do.