It would be nice if there were a federated way to accept "likes" from users (not just friends) across all compatible sites, but I don't think there's a good way to stop a site from Sybil-attacking posts with arbitrary numbers of illegitimate likes.
My instinct is that there needs to be a second layer reputation system for recording which sites can be trusted not to inflate their like numbers. This becomes similar to the problem of which mail servers can be trusted not to send spam, but perhaps if posts and likes are public (unlike emails) then more useful reputation systems are possible.