If you've always used that Facebook account through Tor, and never through your normal connection, and the Facebook account is for an anonymous identity rather than your real identity and you never make any references using your real identity to your alter ego and vice versa, and you never login, then using Facebook over Tor does not necessarily compromise your privacy or security. The point of TAILS and Tor is that they allow you to partition your life into multiple identities in ways that someone knowing one identity won't be able to trace your other identities.
If someone wanted to trace you through your Facebook identity, they theoretically would be able to link your Facebook activities with any other things that you accessed through the Tor sessions that you used that Facebook account on. If you never use the same Tor session for anything that could've been linked to your real identity, then it's very unlikely that anyone would be able to link it to your real identity.
If you're using Facebook through Tor though, you should use Facebook's Hidden Service: https://facebookcorewwwi.onion