You do not need a blockchain to do this. If you can represent something using an append only data structure, a DHT is enough.
Can you elaborate? Is the blockchain not an example of an append-only data structure?
https://medium.com/@arthurb/what-do-blockchains-accomplish-e...
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 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.
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.