Article in NY Times about persona on Facebook compared to real life. I think it's right. Examples:
A 14-year-old I talked to about this sent me a message that pretty much sums it up: “I write more enthusiastically on Facebook than I actually am in real life. Like if I see something remotely funny I might say ‘HAHAHAHAHAHAHAHAHAHAHAHAHAHAHHA,’ when really there is no expression on my face.”
and
It should be said that adults are not necessarily less fictionalized, or more themselves, on Facebook; they are simply less natural, less conversant, less in their element, when they fictionalize. How many people do you know who are in the midst of some great existential or marital crisis, but whose Facebook page is all family photos from the south of France, or the Vineyard, or Bangkok, and charming things their children said?