At the moment most followers seem to be legit, but to be on the safe side you could add some prerequisites to the giveaway like the GRF dev did, e.g. that every new participant on Twitter also needs to have at least a junior member account on bitcointalk, that would counter mass participations by Twitter bots.
Ooooh now i do like that idea but at the same time what if someone wishes to partake but they have never used bitcointalk before or a forum for that matter