Voting would also require an account.

Random internet user would purchase an account if service provided value for them. For example, maybe they can create forums via this system for their gaming guild etc.

Yes, advertisers can purchase an account but there might be a popular moderator who bans these viral advertisements, based on popularity of ad-blocker I imagine such an moderator would get bunch of subscribers. Perhaps moderators can make some money through their subscribers as well, might be an interesting scheme.

Also spot price is pretty strong deterrent for advertisers especially if they keep getting banned through popular moderators.