AFAIK, Facebook is legally entitled to being as inconsistent as they want to be, allowing companies they like to use 'FB', while suing companies they don't like. They are entitled to single out developers that encroach on their territory.
Now the morality of this behavior is an entirely different discussion...
FWIW, I believe that it is more "you can lose it if you lose control of it"; just because you haven't enforced it is not a big deal if the people you haven't enforced it against are sufficiently small that they wouldn't have been noticed in a wide enough audience to undermine your trademark.
(In essence, the argument is not a technical one: it is only interesting in that it changes the dynamics of "could someone have been confused"; if people see the word "Facebook" as a moniker on products all the time that are not made by Facebook, then clearly they are not going to be confused by any individual one.)
I sort of agree: You don't automatically lose anything. But once you sue to enforce your trademark, the court looks at the body of evidence - and may determine that the trademark has been diluted and has been lost. There is no AFAIK predefined threshold (or way to measure, for that matter) the dilution of trademark - a different judge/jury may reach different conclusions.
And basically, that's why most trademark lawyers will act on even the smallest unauthorized trademark use - they do not want to leave it to chance.