Unlike a regular open ad exchange, these private exchanges limit which advertisers a publisher can sell to and which publishers an advertiser can buy from, which can mean bigger-ticket deals than those conducted in the open ad exchanges. They are typically used so that publishers can cordon off their most prized and often priciest inventory for deals with big-budget brands that may span more than a single ad slot, and so advertisers have more control over where and how they're spending their money.
In effect private exchanges look to marry the direct dealings publishers and advertisers conducted for TV and print ad buys with the processing efficiency and targeting that programmatic ad technologies offer.
Facebook bought LiveRail in July 2014 to gain a foothold in automated video ad sales and keep pace with competitors like YouTube owner Google and AOL, which bought LiveRail rival Adap.tvin 2013, as they all jockeyed for advertisers' growing video ad budgets.
Facebook also runs a mobile ad network, Audience Network, that syndicates the ads that brands buy from Facebook across an undisclosed number of third-party apps, including publishers running LiveRail-powered private marketplaces.
If you are looking for a partner to help you with an ad exchange, turn to Look Book.