AppNexus has stepped up its fundraising, bringing in $50 million for its real-time display advertising platform from existing investors as well as Microsoft (s MSFT). The company has now raised $65.5 million over the last three years from investors including Venrock, Kodiak Venture Partners, First Round Capital, Khosla Ventures, Mark Andreessen, Ben Horowitz and Ron Conway.

The company's real-time bidding business counts customers like Microsoft and eBay (s EBAY), as well as eight of the top 15 ad networks. It auctions more than 4 billion ads on a daily basis and is growing fast. While real-time bidding is still a small part of the market, AppNexus said it expects the sector to grow to 20 percent of the display market by 2012.

New York City-based AppNexus, which pitches itself as a scalable cloud service, runs out of three data centers including one co-located at a Equinix facility in Amsterdam to serve Europe and the Middle East.

AppNexus CEO Brian O'Kelley was formerly CTO at Right Media (acquired by Yahoo (s YHOO)), and CTO Mike Nolet was director of analytics there. President Michael Rubenstein was general manager of DoubleClick's ad exchange, then directed the product for Google (s GOOG). Google also recently bought the demand-side platform Invite Media. AppNexus has been discussed as an acquisition target for AOL (s AOL) and Microsoft.