Dice, the terrible job board, has announced a \$100M IPO. Sigh. The answer to many-to-many social network problems lies, not with full mediation, but with semantically-meaningful hypermedia. In other words, semantic Webs (which I use only in the loosest sense of domain-specific tags). At the moment, this isn't apparent, with mediators doing just fine, but as far as investment goes, what you're buying are future cashflows, not today's shiny pennies. To invest in Dice, you're making a bet that either their success at mediation will continue into the far future or that they will be the incubator or acquirer of "what comes next." Given the public face of Dice (i.e., their product), I think that's a bad bet.
That I think we will continue to see transformations in the way we interact with computers leads me to believe that MSFT and GOOG are good investments. I think that Microsoft Research is likely to incubate the future of interfaces (beyond keyboards) and that Google is going to acquire or develop the "business rules"/"autonomous agents" of semantic hypermedia.