Advertisers have been asking this question for the past few years. What is an effective ad campaign in Social Media. What they have found is CTR for display ads in any social site is lower than the already poor display ad CTR on portals like Yahoo. Well then what works? Is it search ads? But there is hardly any search traffic (how many times do you search in Facebook?). So that is not it.
Before we get into a discourse on what works and what doesn't; let's understand the end-user behavior in these sites. What is the user trying to do in any social site - they are interacting - engaging with their friends in discussions, commenting, sharing, etc. In reality social interaction is similar to what we humans have been doing since time immemorial - we interact and engage. And therein lies the answer. The effective formats in social media are the ones that engage and interact the user in some action.
Just look at display ad sold by Facebook to large brands - these incorporate polls within a display ad which can be shared with your friends (Facebook requires an annual brand commitment of $20,000 prior to using these ads). Similarly fan pages which attract an audience must have engaging applications built into it. These don't have to complex, but you need elements of polls, comments, contest, etc. Without these simply broadcasting a brand's message is not going to be effective in Social Media.