Abstract: An online system provides content items including URLs of third party websites to client devices. The client devices prefetch data from the third party website via the URL. The online system controls the prefetch rate for the client devices based on information received from the client devices. The online system may reduce the prefetch rate if an aggregate number of prefetches from the third party website during a time interval exceeds a prefetch quota. The online system may reduce the prefetch rate if the client devices indicate poor performance of the third party. The online system may determine whether to prefetch data from a URL included in a content item based on a likelihood of the user ignoring the content item if here is a delay in loading of the content item in the client device.

Abstract: An online system accounts for measures of quality of content items when selecting content items for presentation to users. For a content item including a link to a landing page outside a domain of the online system, the measure of quality accounts for a likelihood of a user presented with the content item returning to the online system after less than a threshold amount of time accessing the landing page. The measure of quality of the content item is based on the likelihood of the user returning to the online system after less than the threshold amount of time relative to likelihoods of other users returning to the online system after less than the threshold amount of time. The measure of quality of the content item is affected by a difference between the likelihood determined for the user and the likelihoods determined for other users.