Facebook plans to have more than 500 "investors", which means that they will have to disclose their financial results anyways, regardless of whether they remain private or not[1]. Given this situation with respect to financial regulation, you might as well become public since you don't gain anything by remaining private. You could say that Facebook is "forced" to go public against their ideal wishes. Little wonder why they wouldn't be coming to Wall Street begging for money.

[1]Facebook revealed in a document sent to potential investors in early 2011 that it was planning to increase its number of shareholders to more than 500. This will force the social networking site to either disclose a lot of financial information or go public by April 2012 -- which is 120 days after the end of Facebook's last fiscal year[2]. Typically the forced disclosure of financials triggers companies to go public, listing their shares on an exchange and, hopefully, cashing in. - http://www.wired.co.uk/news/archive/2012-01/31/facebook-ipo-...

[2]The 500 shareholder threshold forces companies that have more than 499 investors to divulge information about their financial performance. Although the company may still remain private, it must file similar documents to those of public companies. If the number of investors falls back below 500, then the disclosures can be omitted. - http://www.investopedia.com/terms/5/500-shareholder-threshol...