Neither is alcohol, nor is gambling.

To be less facetious, I don't see why the claim that facebook can't possibly be addictive, is so self-evident that it's a tautology. I can understand people having different stances in the debate on whether it's addictive, but denying that there's a debate to be had seems completely misguided.

(Also, there can be reasons other than addiction for people using facebook despite feeling strongly against it — e.g. peer pressure.)

So, more precisely, your point was actually "anyone who had been feeling strongly about it at some arbitrary point in time will have stopped using it by that arbitrary point in time". Yes, that's a bit easier to justify but still quite uncertain (IMO) and also a much weaker statement.

Also, just because the stimulus isn't a molecule it does not follow that it's not biochemical in nature. Physics and biochemistry play a role all the way down. The very notion of "stimulus" depends on where we draw the murky, fuzzy and ultimately arbitrary border between "us" and "the world".