According to the WSJ article
Microsoft could purchase a stake of up to 5% in the closely held startup, at a cost in the range of $300 million to $500 million
This would value FaceBook at between $6bn and $10bn which seems high when FaceBook expects to have a profit of $30m on revenue of $150m this year, but what do I know?
I don’t suppose there is any possibility that Microsoft are trying to sucker Yahoo! or Google to jump the gun and throw a bucketload of money into FaceBook?