Ben Parr really nails this one. Any consumer-focused product or service needs to keep this in mind.
Why do you post photos on Facebook? Why do you tweet? Why do you check out your Klout score, even though you claim you never check it? The answer is simple: we get an emotional high every time somebody likes, retweets or comments on the things we post.
We always want another hit. Validation is the crack, Internet is the crack pipe and Facebook & Twitter are the dealers.