(In any event, they seem to only want a small stake.)
Last I heard their revenues were less than $50 million. This puts their P/E ratio in the triple figures. The whole business seems too risky. I don't think I would sleep well if I had a lot of my money in Facebook.
Put another way, facebook isn't really focusing on monetizing its users yet - it is (rightly) focusing on becoming the platform of choice for the social web.
10B might be a steal in the long run if you believe a site like this is the future of content distribution or advertising.
I read somewhere else that they suspect that Microsoft just wants the 5% so it has more control over serving ads, much like Google owns 5% of AOL so it can serve a ton of ads to AOL users/visitors.
And there are a tons of reasons that make it clear to a technology person, that bussiness persons will ignore and some will get the money and others lose big.