1. A computer-implemented method of sharing social network information between online social networks, the method comprising: receiving, from a first user over an electronic network, a request for a web page provided by a first service provider and registration information for registering the first user with the first service provider; requesting and receiving, from a second service provider over the electronic network, a unique social network identifier (ID) identifying the first user within a social network maintained by the second service provider; storing the received unique social network ID of the first user in relation to the registration information for the first user; receiving, from a second user over the electronic network, a request for the web page and a unique social network ID of the second user; obtaining from the second service provider, based on the unique social network IDs of the respective first and second users, relationship information indicating a relationship between the first and second users within the social network; and transmitting a modified version of the web page to the second user over the electronic network, wherein the web page is modified based on the obtained relationship information.