I've written about it here:

https://pippellia.com/pippellia/Social+Graph/Navigating+the+social+graph#PoW+endorsement

I think it's a really effective approach because the work can be done by a specialized miner, for a small cost.

"Pay 10$ to get a golden checkmark to protect your audience from impersonators" I think it has reasonable product market fit (see X)