A lot of the ad network and exchange companies were bought by Yahoo and Google for $100+ million basically for the team and not the technology.

Although the "technology" more or less worked, it was discarded and re-written or substantially changed.

In the case of Google, I'm thinking of AdMob.

In the case of Yahoo, I'm thinking of the 2-sided ad exchange network they bought and then ripped out the 2-sided features.