Microsoft has weighed in on this evening’s announcement, claiming that the agreement gives Google (NSDQ: GOOG) 90 percent of the online ad market, and thus makes the market “far less competitive.” So far, it doesn’t have an official strategic response, other than that it’s weighing its options. But it does deliver a key message to Yahoo (NSDQ: YHOO) shareholders: “Our proposal remains the only alternative put forward that offers Yahoo! shareholders full and fair value for their shares, gives every shareholder a vote on the future of the company, and enhances choice for content creators, advertisers, and consumers.” Basically, if you want to get paid for your shares, you’ll go with Microsoft (NSDQ: MSFT). If you want to see the value of your holdings collapse, then sign an ad deal with Google and good luck going it alone. Full statement.

Meanwhile, this should be music to Microsoft’s ears: Senator Herb Kohl (D-WI) has already released a statement expressing concern over the competitive implications of the agreement: “Following closely on the heels of Google’s acquisition of DoubleClick, this Google-Yahoo alliance would represent even further consolidation in the internet advertising market… We must ensure that this consolidation does not foreclose needed competition or harm consumers.” (via Reuters)