From the link:
To be sure, Facebook has focused on increasing its membership, and it has been wildly successful on that score: the number of Facebook accounts is now nearing 500 million. So the sheer size of the Facebook audience is attractive to advertisers and app makers; and as a bonus, Facebook provides data tools to advertisers that help them make meaningful impressions on members of that audience.
But the company is deeply indebted to its venture capital backers, who, while already seeing dividends from Facebook’s current business, are looking forward to a big payday (investment plus return) at some visible point in the future. For now, Facebook’s investors are giving Zuckerberg and company plenty of time. People familiar with the situation say that you won’t see a Facebook IPO this year or next year, but you probably will see one in 2012.