Negative feedback from your peer group is normal, and provides conformity pressures, which is important when forming and maintaining a community bound together by a common culture.
In an in-person peer group, there are a lot more subtle gradations in disapproval. "Dude, that's not cool!" is very different from looking at someone askance. Overt signals require everyone else present to take a position on something. Subtle ones don't.
Being downvoted on a Facebook peer group of teens is an overt signal. Everyone is going to take sides. It's not that it doesn't happen in person, it's just that online it makes everything into a Big Deal. That produces bland communities with fear-based conformity.
To do better, you'd need something that can support richer communication, allowing for more nuance. One possibility that comes to mind is emojis -- a group can (and does) form its own vocabulary based on emoji-only responses that may only have meaning within a group. But they can serve the purpose. If I were a teen, or more active in social forums, I might know if that's already happening.
Extreme peer pressure can be bad, just as any social extreme is bad. But it isn't universally bad if used sparingly and for a reason.