So why does Microsoft think Facebook is worth $15bn? Here are 15 possible reasons....
1. The network has gone viral in the last 12 months, with more than 50 million users worldwide and a user base that is growing faster than great rival MySpace. According to Facebook, it adds 200,000 new users each day.
2. The average user spends 3.5 hours a month on Facebook - more than the average user on rival MySpace - which is increasingly attractive to advertisers.
3. Facebook is the current Web 2.0 darling - popular with ordinary users and tech heads alike.
4. US research reveals that Facebook users come from wealthier homes and are more likely to attend college than MySpace users - increasing that attraction for advertisers.
5. Microsoft's investment makes them a serious player in the growing market of social advertising. Social network profiles are full of personal data that users voluntarily hand over, which is very useful for targeting adverts.
6. Sixty percent of Facebook users are outside of the US - so Microsoft's investment buys access to a global audience quickly and simply.
7. Facebook is the new web: The decision to open up the network to outside developers turned Facebook into a destination for many uses, like messaging, photos and video. Of course, as Facebook is on the web it could never really be the new web.
8. Every major content firm with an online presence is either working on a Facebook application or has already launched one - from Google to the BBC.
9. According to a report, 233 million hours of work are lost each month in the UK due to staff looking at social networks. Advertisers can now target people when at their desks.
10. The openness of Facebook is attracting a wealth of talented developers who can launch their applications to millions of users quickly.
11. Facebook messaging is the new e-mail. Everyone feels stressed from a deluge of e-mail from unwanted people and companies. But Facebook messages are always from friends.
12. Facebook's status updates have become the easiest way to let friends know what you are doing and how you are feeling at any given moment.
13. Facebook thrives on playful applications such as Pirates, Zombies, Super Wall and Top Friends, which have made the network a place to play as well as communicate.
14. Facebook is the acceptable face of blogging - you can reflect your life and personality online without being seen as a blogger, which often carries a geeky stigma.
15. Facebook is worth $15bn only because Microsoft says so. The value of Facebook is based on a 1.6% share of the firm being worth the $240m Microsoft paid for it. Microsoft and Google were in a bidding war for a slice of the firm and both companies have large pockets. This was not just business, this was personal, according to some analysts.
Do you have a facebook account?