Social networking site like Facebook Video sharing site like Youtube Free blogs hosting like Blogger Music sharing like Playlist Dating site like Match.com Above are membership (community) sites which are not online retail stores. Compare them to membership sites that are online stores like Amazon.com , ebay. Which is easiest and fastest to generate revenue?
Celebrity and niche sites are very easy to monetize. However, it sounds like you are looking at social networks more than membership sites.
All membership site can be easily monetize I guess especially traffic type of membership.. or a content membership sites etc.
From your list, the best ROI is dating but, by far, not the easiest. Video sharing can be profitable but you'd have to have a niche (think adult maybe?). Music sharing, legalities are an issue especially after napster. Facebook/SN sites, I don't believe FB has ever made a real profit only paper, so that's probably not going to fly.