I think in light of the fact that government agencies probably capture more data on innocent citizens than they want to admit has brought Facebook to this point.
My belief is that Facebook will use my personal data to sell me things. That is their whole strategy. And even though the Guardian says the US government can detect Tor users, Facebook still wants to cater to a privacy-oriented crowd. Even so, what is using Tor with Facebook accomplishing? Hiding your location? That's about it.