The community equivalent of Proof of Stake is reputation. You can't do anything until you've shown up for awhile and contributed in small ways, you gradually gain the ability to contribute in bigger ways, and if you are discovered to be malicious or corrupt or toxic, then your rights are revoked. The people who've gained this trust are presumably motivated to maintain it (although there's always the risk they sell their account/identity/soul for healthcare and do some damage before they're found out).