If you assume that Facebook will comply with whatever entity you're trying to avoid, by using Onion, it sounds like a pretty bad idea to me, yes. In theory you could probably use a separate browser and Tor session for Facebook, and for your other browsing - making it a little harder to associate your Facebook login with your Tor session (ip). Sounds like a terrible idea, though.

Now, for some of the reasons why you'd want to use Facebook via Tor, it might not matter much - using Facebook might be bad enough (eg: it could be considered subverting state censorship) -- so if Facebook is already colluding with your adversary, just having a Facebook account might be enough to give you problems.

It might be enough for a legal veneer of plausible deny-ability, although I doubt it: Eg, perhaps you're a drone pilot and you login to Facebook via Tor, and paste in a gpg-encrypted, ascii-armored text-message to a journalist on Facebook. You could claim someone must've hacked your account. Or you could collude with someone else, and "borrow" their account. I don't think it'd keep you out prison though.