Not much. Researchers gave personality tests to a bunch of people and then had other people, who didn't know the subjects, estimate the subjects' personality based on their Facebook profiles. They found that the impression people get about a person from that person's Facebook profile matches the personality test. So if people are trying to portray themselves as different than they really are, they're failing. Or maybe Facebook is now just another way of communicating.
The accuracy was greatest for extraversion and lowest for neuroticism, which the authors say is notoriously hard to infer except in person.