You need a Sybil prevention mechanism to avoid stuffing the ballot box, and that can be a small proof of work stamp, but that doesn't mean you need to use a blockchain and incur the scalability hit.
Votes belong to each user and are spent exactly once. You could try to vote twice but it would be detected and would not lead a double spending problem.
I agree that you don't need a blockchain, but can you elaborate?
I think proof-of-work is a bad mechanis to prevent sybil attacks on any new blockchains except bitcoin because someone can just cause a massive jump in computing power for a dayand destroy the whole network. Now the infrastructure is there, we need proof of stake or something else.
1) What technology is appropriate for a decentralized reddit, and
2) What technology is best for a decentralized cryptocurrency.
Regarding (1), a small proof of stake stamp for upvote is fine. So, someone can upvote a Lock Ness monster comment to astronomical level for a while by buying computing power? Big deal. Or you can build a social network and rely on something like Sybil Guard. You don't strictly need a consensus here... you can decide to only view the upvotes associated with people in your extended network.
The reason they are using a blockchain is because it is sexy, and it is easier than thinking seriously about the problem.
Regarding (2), proof of stake with safety bonds and yearly checkpointing is probably the sweetspot.
Right! That's why I think fully distributed peer-to-peer is the wrong goal to strive for. A lot of group activities (like posting to a topic and voting) are much more easily done in small cohorts. Yes, mental poker is possible but it's in its infancy and our efforts are currently better spent assuming there are going to be servers. The question is, can we choose what servers! The DHT can route you to a server which is authoritative for a particular group activity. That allows you to clone some post and share it with your friends (on a server you trust) and they can reshare on servers they trust ...As you said, what is needed here is not consensus but just signaling from people you KNOW.
The rare resource here isn't work, it's reputation. You personally won't add 1 million friends. If you want to get signals and votes from friends then you don't need proof of work.
Votes belong to each user and are spent exactly once. You could try to vote twice but it would be detected and would not lead a double spending problem.