2. The computer-implemented method of claim 1 , further comprising: subsequent to detecting the user-initiated interaction with the web document and responsive to determining that a copy of the web document has not previously been analyzed to identify topics to which the web document relates, requesting and receiving a copy of the web document from a content source identified by the URI, wherein the identifying one or more topics to which the web document relates is performed based on a textual analysis of the web document.
2. The computer-implemented method of claim 1 , further comprising: subsequent to detecting the user-initiated interaction with the web document and responsive to determining that a copy of the web document has not previously been analyzed to identify topics to which the web document relates, requesting and receiving a copy of the web document from a content source identified by the URI, wherein the identifying one or more topics to which the web document relates is performed based on a textual analysis of the web document. 3. The computer-implemented method of claim 2 , wherein determining that a copy of the web document has not previously been analyzed to identify topics to which the web document relates includes determining that the URI is not stored in a cache or database accessible to the social network service.