Re: "...no mechanism to prohibit...from a single IP address"
> I'm not an expert, but (as I understand it), NAT (particularly for mobile connections) places numerous devices on a single public (routeable) address. So insisting that each user have a different IP address would block many legitimate users.
And the problem is...?
Another anti-bot technique is to impose rate limits. Limiting Facebook, Twitter, Instagram etc users to, say, one post per month should solve any bot problem.