Anybody not on them still gets to vote. If anything, I suspect that making these platforms increasingly unrepresentative will end up hurting pols who pay attention to them, by giving them an increasingly distorted view of public opinion. If you optimize your messaging for Twitter, an actual electorate is going to drop you like third period French.
People will trade off convinience and representativeness. A lot of psychology studies are famously done on students, because it's just so cheap and easy. I think it's very likely they will just say "screw those backwards hillbillies, I bet they are all racist unpersons anyway"
