interesting Slate column discusses recent research on Facebook and its impact on our own attitudes, in particular whether seeing all those shiny happy people makes some folks sadder. It makes sense. Rarely do we post to our newsfeeds the bad stuff, so for some people, seeing this constant feed of happiness, this may result in them reacting negatively.
In general research shows we project more happiness on others than really exists, so the FB thing just feeds it further. The power of projecting such happiness on others, in other words, can influence our own emotional state.