I agree with your basic sentiment that social networking is driven by the tenor and tone of your real life friendships and interests. However, it differs from real-world socializing in that it is a broadcast model rather than a conversation model. When you and I meet for coffee, the cost of telling you about my new 19t Surly steel cog is, shall we say, two minutes. We only have so much time for coffee, and I ought to spend half of it listening to you, so am I going to decide that the cog is worth two minutes of our limited time to tell only one person about it?
But online, I appear to have unlimited time to broadcast what I want to say. If I spend two minutes on the cog, it takes away from two minutes of the Real World, but it doesn't appear to take away from our limited time together, and I blast it out to all my friends, some of whom may care.
I believe this gives me an incentive to talk about fairly trivial stuff that doesn't exist in the face-to-face world.