It's just how I see it is if someone sees this and wants to do it, they'll simply follow you till they either win or lose, then abandon you as they don't really care about your content, they were only here for the giveaway.
If I did one I'd make a list of all my followers and restrict the winnings or heavily be biased to them for entries.
So either only previous followers can enter or they get 5x the chance of winning or something.
alright that's a interesting idea i have to say... I will probaly use that for my next give away :) thanks for sharing