You are correct, that would be a risk for them (especially for FB) because of how big they've become.
Anecdote: in 2014, I registered an account on Twitter called "tweetcoin". I felt social networks would start developing their own currencies. I posted one comment, "exciting things in the works". I was promptly locked out of the account, and a new tweet appeared there (dated 2012!), stating "this is not your account."
You can see it here: