I would agree with the comments above regarding "One ring to rule them all" (i.e., one "Social Network" site or your own website provides a base reference for the others). This is exactly what I ended up doing quite a while back. now. Some years back, I started using RealContacts and then LinkedIn, because they were relevant to business (not social) networking.
The thing is, social networks take time, are of questionable use, and maintaining your profile and persona on them uses up your cognitive surplus. I therefore did not follow all the invitations of friends/acquaintances to join Plaxo, Where-are-you-now, Facebook, etc. I recently joined Facebook (after it had sorted its gaffs in data security out) and set up a personal profile there, which linked to my professional profile on LinkedIn.
I later joined Plaxo, leaving my profile empty except for links to my personal profile on Facebook and my professional profile on LinkedIn.
This seems to work for me, but it still takes time and cognitive surplus to maintain these things - tasks which I did not "have to do" before.