1. A method comprising: by one or more computing devices, for each information item of a plurality of information items in a collection of information associated with a first user: constructing a vector space model; and adjusting the vector space model by an importance score of the information item, wherein the importance score indicates the level of importance the information item is to the first user; calculating a degree of similarity for every two information items in the collection of information; based on the degree of similarity calculated for every two information items in the collection of information, clustering the plurality of information items in the collection of information associated with the first user to generate a plurality of information topics using the adjusted vector space model of each information item; for each information topic of the plurality of information topics generated from the collection of information associated with the first user, determining an interest score, which indicates a level of interest the first user has in the information topic relative to other information topics within the plurality of information topics; for each information topic of the plurality of information topics, determining a second interest score, which indicates a level of interest a second user has in the information topic relative to other information topics within the plurality of information topics; determining that the first user shares a common interest in a plurality of common-interest information topics with the second user, wherein the determining that the interest is a shared common interest is based on the difference between the first user's level of interest in an information topic and the second user's level of interest in an information topic, as determined by the interest score and the second interest score respectively, being less than a threshold difference; determining a common-interest strength between the first user and the second user, wherein the common-interest strength is calculated based on: the number of common-interest information topics shared between the first user and the second user, a respective level of interest, as determined by the interest score and the second interest score, by each of the first user and the second users in the plurality of common-interest information topics, and a correlation and difference between the respective level of interest in the plurality of common-interest information topics by the first user and the second user, the common-interest strength measuring the strength of connection between the first user and the second user; and establishing a connection in a social network between the first and second users based on the common-interest strength.
6. The method of claim 1 , further comprising ranking the plurality of information topics based on the interest score of each information topic.