I think it's moreso the other way around. Nobody cared much about them until they were casted for Twilight movies. People were into Twilight and the idea of a movie before anyone was casted.
I'm sure it contributes some, but the people who are into Robsten are major Twilight fan anyways, so I don't think it has that much of an effect. Some other fan don't even like the casting.
(There are a lot of people who watch a lot of film just for the casting. I'm sure there are a lot of people here who watched "The Woman In Black", that Ballet Slipper movie, and "Cherrybomb" just because of specific cast members.)