FB has been going up quite a bit. There are a lot of people who bought in the area of around 33. Many of these people have held their shares underwater for 6 months. They are just getting out. It is just profit taking. 33 (plus some change) is quite a powerful resistance. It wont hold if there is a really good earnings report.
Two simple reasons:
1.) The longs who had it at under 20 wanna make sure they profit before earnings just in case the chance of bad performance/outlook. These guys I am sure also just bought in right after they sold too.
2.) The big sharks want to drive out the little guys like you and me so they can buy at every dip in preparation for excellent quarterly results/outlook.