AppNexus, a company that provides online advertising via a cloud computing platform, has secured a $5 million third round funding as it looks to capitalize on the growth of real-time ad buying systems. In addition to building up its marketing efforts, AppNexus President Michael Rubenstein (pictured, left), who joined two months ago from Google (NSDQ: GOOG), tells paidContent that the company plans to use the proceeds from the funding to make some more high profile hires.

This latest round was led by Kodiak Venture Partners, which was joined by previous backers Venrock and First Round Capital. The New York-based company raised an $8 million second round last year. Before that, AppNexus received roughly $2 million in angel funding from Ron Conway and other individuals.

AppNexus CEO and co-founder Brian O’Kelley, who was previously at real-time ad exchange Right Media before it was acquired by Yahoo (NSDQ: YHOO), said in a statement that the timing is right for AppNexus’ services, as more advertisers are turning to the real-time auction model.

Aside from the more established Right Media, Google has ramped up DoubleClick’s display exchange by fully linking the AdWords system to it. Also, Microsoft (NSDQ: MSFT), which bought the ad exchange AdECN, recently struck a deal with open-source ad server OpenX Technologies.

Rather than view itself as a competitor to those systems, AppNexus wants to be seen as a “gateway” to ad exchanges and ad inventory aggregators. In a conversation with paidContent, Rubenstein said that AppNexus doesn’t view itself as a competitor to ad networks or the other exchanges run by his and O’Kelley’s former companies. “This eco-system has a lot of room for a lot of different players,” he said. “I’m very bullish on ad networks and I don’t see exchanges as an evolution of that kind of service offering. But I do see a role for a company like ours that can connect advertisers with exchanges and ad nets. This new funding will help us support that position.”

• Industry Moves: Former Google/DoubleClick Exec Rubenstein Heads To Cloud Computing Ad Firm AppNexus