Microsoft's recent investment into Facebook wasn't surprising in that it was done, but did surprise many (especially analysts) about how much they were willing to pay. For the relatively small share of 1.6%, they handed over $240 million, which gives the company itself an estimated value in the billions of dollars. From an objective point of view, many are seeing this as a waste of Microsoft's money and have said as much.
Microsoft doesn't feel that way. Even if they did overpay, they think that the investment was an important strategic maneuver and one that was well worth the cash. Ballmer claims that the value of Facebook may be up in the air, but their goal is more to create a "pre-eminent" presence in the market. I think it's a little late to create a "pre-eminent" presence, but I understand the logic. Then again, if the business decision was a sound one you'd think they wouldn't have to defend themselves in the first place.