1. A computer-implemented method of assessing the suitability of particular key phrases for use in providing contextually-relevant content to users, the method comprising: identifying a key phrase that appears on a page of a site; and generating a score for the key phrase based at least partly on view counts of social media content items associated with the key phrase, said social media content items being accessible to users on one or more social media sites that are separate from said site, said score representing a suitability of the key phrase for selecting contextually relevant content to present on the page, wherein generating the score comprises assessing, based at least partly on the view counts of social media content items, a rate of change in a popularity level of the key phrase; said method performed by a computer system that comprises one or more computers.