Nice interview with Martin Sorrell in the FT over the weekend where he argues that the valuations of online media companies defy economic sense. I heartily agree - Facebook in particularly seems to have attracted a "bubble mentality" amongst investors who should know better. The $50B valuation is ludicrous - my own estimates, based on past performance of the successful dotcoms, the addressable market for social media and what we know about Facebook's financial performance suggests an upper bound of $13B for the company.
Time will tell whether this is bourne out by the market when (or if) Facebook finally IPOs. I personally feel that comparing them to Google is folly because of the position they occupy in the value chain and the proven strength of the Google team both historically and at present. We shall see.