I'll admit, on /r/HongKong the "unify with mainlanders" posts don't rise up. It is not trendy. Probably because a lot of people interact with mainlanders that are against their movement. Which my understanding is that this is a selection bias (mainlanders that support them aren't seen). But this is going to happen in any movement that has a lot of fear in it (there is clear justification for the fear).