Social media means very little.
Cher had a huge following last year yet her voting percentages weren't great overall (compared to Matt, Rebecca, Mary and 1D). Also, Frankie had the most followers on Twitter at one point and he wasn't popular with the voting public.
I doubt Little Mix are topping the vote (although they may have done last week) but I think they are a certainty for the final. And once they get there, anything could happen. The hype that they are the only girlband ever to get to the final will encourage a huge amount of votes from floating voters.