Occasionally you bump into people you know. You didn't come to the mall to interact with them specifically, but hey they're your friends and they may have something to say, you may have something to say, so you chat for a bit.
All the while, you're there for an alternative purpose. You have to pick up a gift for someone, some tools from sears, a couple bags of socks from Macy's. Maybe you just wanted to look around and see what each store has on sale?
Your friends may choose to tag along. You may each follow each other around to the various stores-- since you have one thing in common: You're both at the mall. You're there to shop.
But, your friend may have somewhere to be at 3PM. You may have just come on for a quick half hour jaunt and then you're off again. You may be in a hurry, maybe there is just one thing you wanted to get and then you wanted to leave.
It's not intended as a slight against your friend. You didn't call him up and say hey, meet me at the mall, and then proceed to power shop your way around and then say 'Gotta run ... c-ya.' ...
At any given time, either of you may have other priorities that come up. Since you didn't come there specifically to hang out with each other, logic has it that you can't apply much vested interest in doing so-- unless, during the interaction .. you decide to both go get something to eat, see a movie, whatever. But that requires negotiation, and re-prioritization. It may not be an option. It certainly isn't the default.
I think it's important people keep this in perspective when quantifying online interactions.