1. A computer-implemented method of selecting one or more key phrases to use to select contextually-relevant content to present, the method comprising: identifying a key phrase that appears on a web page of a web site; generating a score for the key phrase based at least in part 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 web site, wherein generating the score comprises determining, based at least partly on the view counts, whether a rate of consumption of social media content associated with the key phrase is increasing; and using the score to determine whether to use the key phrase to select contextually-relevant content to present on the web page; said method performed by a computer system that comprises one or more computers.