A study suggests that social networking profiles reflect an accurate, not idealized, version of users' personalities. The study author says it's "because people aren't trying to look good or because they are trying and failing to pull it off." [EurekaAlert]
I have to believe it is the first one. If someone was trying to look good and not pulling it off I don't think it would be an accurate reflection, it might not be an idealized one but it still wouldn't be accurate.
I think the thing about facebook is many of the people on it will also know you in person. Hence the authenticity. Now if it was just people you'd never see in real life, I think it would be quite different.