First off, no one ever puts a significant amount of outstanding shares up for an IPO. You really think the pre-IPO owners, including Zuck and partners, would give up control of Facebook during an IPO? There are well over 2+ BILLION fully diluted shares out there, if not 3 Billion after this IPO.
Valuation down to about $89 Billion. Yeah, so, at $30'ish, that's about 3 Billion shares out there.