The word on the Street is that more and more people are using Facebook through mobile phones which have fewer ads compared to those seen on a computer. Therefore Facebook isn't selling as many ads and as mobile phone usage grows and computer/laptop usage stays stagnant, that effects growth.
I've always thought the Facebook thing was overhyped. Facebook can only be used against you. Facebook has already been used to convict people of crimes, fire people, and deny people jobs and positions. Your friends, your real ones, already know what you're doing and what you like. There is no need to advertise yourself to them. Whatever you post on Facebook can be and will be used against you someday, with devastating consequences.
The only people who need Facebook are people who need to advertise themselves and create some sort of image/brand like celebrities and politicians not to mention companies. Therefore Facebook is becoming more and more managed like a PR department. The smart people I know make sure only stuff they want to show gets put up on Facebook. It is totally fake. Them drinking, taking drugs, you'll never see that, only them being professional and with their "happy" family all smiling and laughing. Again, if you're a real friend, you don't need Facebook and you'll laugh at the Facebook pages because they are so fake.