If we look at sites like Facebook or Linked-in or any company building social websites/applications chances of finding number 2 is less. I haven't heard any Linked-In or Facebook competitor.
We do see number 2 like Bing, Yahoo etc because they are not startups. Bing is funded by Microsoft and Yahoo was number 1 sometime in the past and has enough money to self-sustain for a while. How long Yahoo will survive?
Tagged is a number three, not owned by a large corporation, not a previous giant.
When it comes to social networking, network effects normally result in one or two highly visible giants and a long tail of smaller, likely specialized networks. These can still be profitable, and of course there's more to the web and startups than stuff that is social in nature.
We do see number 2 like Bing, Yahoo etc because they are not startups. Bing is funded by Microsoft and Yahoo was number 1 sometime in the past and has enough money to self-sustain for a while. How long Yahoo will survive?