I'm getting regular invites for professional social network, Naymz, but I ignore them since I already have a thousand contacts on LinkedIn.
Of course, there are winners and losers in any medium, and, as Friends Reunited showed, a winner can rapidly become a loser. However, the overall problem is that there are only so many hours in the day.
And the reality for the English speaking web is that the online marketplace is reaching saturation; any growth has to come from eyeballs moving from other leisure activities such as gaming and TV viewing. The 'growing pie' has become a 'shifting pie' and everyone's clamouring for a bigger slice.