If enough of their users ask for it, they have nothing to lose by integrating it. They wouldn't have bought instagram otherwise. There's no particular reason for them to champion it so they'll probably be laggards but enough demand may bring it along eventually.
So I agree, that Bitcoin may be only added if there is enough pressure from the users to do so, the question now is, whether such appears? For that, it is needed to have a bit more positive picture of Bitcoin in the main stream media, first.