Microsoft (Nasdaq: MSFT) has made its third digital ad acquisition in the last two months: first Screentonic (on mobile advertising), then the big one with aQuantive, and now it has bought online advertising exchange AdECN, which it says adds a final element in its portfolio. Financial terms were not disclosed. AdECN, based in Santa Barbara, CA, will be integrated with the company’s adCenter platform.

AdECN is a competitor to Right Media, which was recently bought out by Yahoo…it was founded in 2003, and has roughly 30 employees. It runs an exchange where 37 ad networks, which purchase ad slots from many different sites and sell them at higher rates to advertisers, buy and sell ads.  It had raised a $3 million bridge round of funding from Novus Ventures in 2005, and $1.5 million in first round financing from Accel Partners last year. More info in release.

• Microsoft Acquires French Mobile Advertising Firm ScreenTonic
• Microsoft Buys Online Ad Shop aQuantive For $6 Billion