Avoiding Facebook (NASDAQ:FB) advertising is about to get harder. The social network is undertaking a test that places ads in users' desktop News Feeds.

"Allowing advertisers to reach people in News Feed is important because people spend more time in News Feed than any other part of Facebook. We also believe that ads delivered through [Facebook Exchange] will create more relevant ads for people," the company said last night in a blog post.

Facebook introduced Exchange, a tool for targeting ads based on browsing history, last fall in hopes of  displaying more relevant pitches on the right side of the desktop app screen. The new system allows ads to be placed both on the right side and contextually, in Facebook News Feeds.

For now, Facebook says it is conducting a "closed alpha" test with a small number of demand-side advertising platforms, including TellApart, MediaMath, and Nanigans. The company says it plans to bring in additional partners in the coming weeks.