1. A method comprising: crawling, by a search engine of an online social network, a first webpage of a first web domain, the first webpage comprising one or more links to one or more second webpages, the one or more second webpages being within one or more second domains, wherein each web domain is external to the online social network; accessing a domain ranking for each second web domain, wherein for each second web domain the domain ranking is based on one or more domain-quality signals associated with the second web domain, and wherein at least one of the domain-quality signals comprises a measure of activations of one or more social plug-ins of the online social network available on one or more webpages of the second web domain, the social plug-in being an executable script providing an activable user-interface element for interacting with the online social network from the webpage of the second web domain; selecting one or more of the second webpages to crawl based at least in part on the domain rankings of the second web domains associated with the second webpages; and crawling, by the search engine of the online social network, each selected second webpage.
1. A method comprising: crawling, by a search engine of an online social network, a first webpage of a first web domain, the first webpage comprising one or more links to one or more second webpages, the one or more second webpages being within one or more second domains, wherein each web domain is external to the online social network; accessing a domain ranking for each second web domain, wherein for each second web domain the domain ranking is based on one or more domain-quality signals associated with the second web domain, and wherein at least one of the domain-quality signals comprises a measure of activations of one or more social plug-ins of the online social network available on one or more webpages of the second web domain, the social plug-in being an executable script providing an activable user-interface element for interacting with the online social network from the webpage of the second web domain; selecting one or more of the second webpages to crawl based at least in part on the domain rankings of the second web domains associated with the second webpages; and crawling, by the search engine of the online social network, each selected second webpage. 12. The method of claim 1 , wherein at least one of the domain-quality signals comprises a measure of engagement with content associated with the second web domain on the online social network.