I'll keep this short, since I've got to jump to other things. But please check out the article in today's Times, which details all of the activity around Facebook since Microsoft let it be known that they are considering at $500MM investment in the social networking platform. Yes, FB is a closed network, but the article notes that there have been more than 4,000 applications created for the platform. I ask again: Where are the brands who are creating custom applications here? At least one entrepreneur has developed a tool that helps determine monetary value to the applications. So, if you notice that your valuation is growing, that's one indication of growing engagement.
Another strategy might be to simply buy applications as they reach a certain usage threshold, in which case you'd (ideally) be buying the user records. One company already did so.