I don't work at either, but know a lot of people at both.
Anecdotally, both sets of people really like their jobs a lot.
I think the people at Facebook feel they have more structure. This is desireable for some people, less so for others. But we're not talking about too much structure, just a sense that things are a bit better organized at Facebook.
That being said, everyone I know at Google seems much happier. The company seems to place a high priority on building the self-esteem of its employees, and reducing the many minor humiliations that are part of working for anyone especially a big company.
The people at Facebook are treated well too, but they all seem much more stressed out.