DoubleClick doesn't use MongoDB (afaik). My understanding is that it is switching over gradually to internal google infrastructure services - so presumably bigtable like things, and its internal successors, whatever those are. That said, MongoDB is basically what I wish I had had when we began building DART long ago.

In the early days of DART we build a proprietary distributed key/value store called UIS. It was super simple and super fast and worked well. The request volume, both read and write to it, was huge: it was hit in realtime by the ad servers.