The exchange only features standard display ads, not sponsored stories or premium ads. Thus users can't buy Facebook's mobile-only sponsored stories, also launched recently.

Here's how a Facebook spokesperson described the new service to us:

Today we announced the Facebook Exchange, which will enable real-time bidding on Facebook. Real-time bidding is a process that lets advertisers bid in real time on a specific ad impression. This means that advertisers can deliver more relevant advertising in a timely manner at a scale not possible before.    

There are numerous existing third parties that help advertisers to do this across the web today. By bidding on a specific impression rather than a larger group, advertisers are able to show people more relevant ads while also running more efficient and effective campaigns.    

For example, a travel site may be interested in reaching a person who searched for a flight but did not complete the purchase. With Facebook Exchange, this travel website can show that person a related ad on Facebook.     

We do not share any user data with advertisers and people still have the same control over the ads they see on Facebook that they do today.