Here at Bullpen we always look to see who is on deck. After Demand Media, the lineup for Social IPOs includes LinkedIn, Pandora, Zynga, Groupon and eventually Facebook. Next up to bat is LinkedIn, which is now trading at a $3B valuation. Yes, trading, even though it is not yet public. Markets fill gaps, and a secondary trading market for private shares has evolved for these social IPO candidates.
Demand for these shares is hot hot hot, and Facebook ticked to a $70B valuation even as Goldman Sachs did an organized sale of $1.5B at a $50B value. Goldman marked Facebook up a bit, and it was peddled for around $55B. Since then, Facebook shares have drifted higher, back to $70B, even amidst a general sell-off in stocks.
You might wonder why they have to subject themselves to Sarbanes-Oxley and other nefarious regulatory schemes when they can raise and trade effortlessly in private markets. Well, the Empire eventually strikes back, and the SEC has put pressure on Facebook to go out as its tradeable shares close in on the 1934 Act limit of 500 shareholders.
Until the Empire shuts it down, SecondMarket is a great resource. Here is its current hot list, showing which social IPOs should be hot:

[...] My view of the LinkedIn IPO pop is that it reflects neither a bubble nor a one-off, but the depth of the pent-up demand for tech IPOs that has been suppressed over the past decade by Sarbanes-Oxley and the over-reaction to the dot-com bubble (eg. the demonization of stock options). We have seen the swirling desire for getting into the Social IPO candidates in the robust SecondMarket trading. [...]