My approach is to have largely disjunct sets of social networks. You're a work colleague? Fine, I'll add you on LinkedIn and Google+. You're someone I met socially and you want to link up on Friendster? Fine.
It does go both ways, I know a lot of people who socialise in a work-like way. They have a small business, they're Amway victims or whatever. If I don't see them as related to my work I will keep them on the social side. Or the black-list, if that's how I feel about them.
I have cross-over stuff, like selling photos and hanging out with photographers, so I run multiple streams/sites for my photography. I generally add those people on both career and social sites, but I also partition them within the site. I have facebook groups for "family", "friends" and "clients", for example.
So my answer to those requests would be "why don't we use LinkedIn" (or whatever).