So here's my deal:
LinkedIn: Have over 1,000 connections, use it as an online address book. Link with friends, colleagues, professional acquaintances so I can find them when I need to. Level of contact with most people: Low.
Facebook: Have over 1,000 connections, more on the friend side, but also including professional colleagues. Like to use it as an address book, but especially love the wide-ranging status updates and seeing what people say and do. Interaction level is high, both broadcasting updates and communicating with specific people 1:1.
twitter: I have maybe 100 people I follow; half that who follow me, and my focus is friends--i.e. I have spent 1:1 time with you in the real world and I care whether you brushed your teeth or had trouble at customs (at least a little). I am not reading you so you can market to me. In other words, interaction very high, but my use of it closer to a closed circle than a porous network.
Or, it's the conversationsphere, baby.
Or, to spell it out: LinkedIn is the broad circle, FB the known universe, and twitter a smaller group. If I had more non tech friends on twitter, I'd focus on them,
Or, in other words, it's not YASN (yet another social network) it's LOSN (lots of social networks), each with their own hierarchy of value and exchange.
(And if I were the proprietor of any social media property--or even of a social search tool--I'd be reading this post right now and asking myself what spheres I'd want my product to fall into for the user and how I might present features and experiences that would support that goal.)
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