as far as I know, until now there are no other altcoins that are a bitcoin threat. bitcoin is still a bitcoin, it cannot be compared with Facebook coins. then how can you speculate that Facebook coins are a threat to bitcoin? while they are still new and not even released in the public. I think if later Facebook coins appear more influential on user adoption.
But I think it won't be that influential. Though we can't say that Facebook coin is a threat for Bitcoin but it possibly be considered as a competitive coin in the future. We know how Facebook itself known to everybody and some instances that people will choose this base on popularity. ut taking into the account of being a threat with BTC, I've never seen it.