As long as 당신 know who they are and you're 100% sure they aren't the same gender as 당신 (Unless your lesbian. If that's the case, opposite gender) then I suppose it's okay. Just as long as they aren't a creep 또는 something. At least know what they look like in real life JUST to be extra safe.
No there are a lot of couples who are both on fanpop.
However, if 당신 have not met each other - to say 당신 are in 사랑 with someone on 팬팝 may be a bit weird. 당신 really don't know the person, and he / she may not be the person they are pretending to be.