Although these may not be considered as pure social networks, special attention should be given to the social sites which are dedicated to vertical markets such as the travellers community (for example tripadvisor, travbuddy or ex-dopplr), films or music. These sites offer services opinion sharing by the users and they compute rankings. For example, ex-dopplr allowed its members to register their personal and business travel plans and get alerts for friends in same places, travel overlaps, or get travel advice from other travellers. One of the open issues is to establish methodologies and tools to distinguish between real and fake opinions on those social sites, which is applicable to social networks in general.
Another relevant social opinions exchange tool is the “social bookmarking”, which main site is del.icio.us with 9 Mio users who share and establish collections of bookmarks between users.