Mark Andreessen says Facebook will bring in over $500 million in 2009. 

Sure, $500 million is a lot of Money.  But Facebook is a new media to advertise on.  With advertisers reporting CTR (Click Thru Rates) of .4%, the money wont be flowing into Facebook for long.  Average e-mail ad campaigns (spam) yield a 1% CTR. 

By now it is blatantly obvious that Facebook's data isn't structured in a way for it to contextually serve ads.  I am 24 and I get life insurance ads on a daily basis, or I am engaged and I get dating ads.   How long will advertisers pay to display ads to people whom:

1) The Ad isn't relevant.
2) Don't Care/Ignore Ads.

Unless Facebook drastically improves their Ad serving platform I don't see revenue growing substantially.  Mark Zuckerberg and others at Facebook have said that the company is in a growth phase, not a monetizing phase.  But you need to tap your users for revenue while they still care.  Case in point, a lot of people I have talked to check Facebook daily, just like they check their e-mail daily.  Once they see that nothing new has happened that impacts them, they log off.  A far departure from a year or two ago, when these same people checked Facebook every 10 minutes out of compulsion. 

All of this is enough to get Facebook to an IPO.  However, once Facebook is a publicly traded company, growth phase will not be an option.  Investors want profit, plain in simple.  Remember when Google was a cute start up?  Then an IPO and the "Do No Evil" culture slowly, then quickly evaporated.  Next comes the exodus of talent, then a complete lack of innovation.

Facebook now employs roughly 1,000 employees, which if you figure pays them $100k, that is $100m a year in salary expense.  That isn't including payroll taxes, which could be as much as another $10m.

It has also been reported that for every 1 million users, Facebook spends $1 million on servers, networking equipment, etc...  As of April 2008, Facebook had 10,000 web servers, 1800 Database servers, and 800 caching servers. Since then, Facebook's number of users have more than doubled.  What does all of this mean?

A lot of hardware to maintain, power, house, etc...  As growth slows, and user engagement drops, they'll start to gather dust.   Granted, they do lease a lot of hardware, but that is a lot of upfront expense on the bottom line.  How will Facebook pay for all of this equipment, keep hiring people, and have lush offices with many perks?

Another laughable thing about Facebook is that their product changes have turned it into a feature rich Twitter.  While the purpose of Twitter is to keep it simple, 140 characters at a time.  Twitter is also a better place for a business, customers choose to follow you, and you have a direct line to them.  For Free.