Another source of stickyness for social web sites
Recently, I've started actually doing things with my Facebook account, and I've done a bit with Twitter, and of course I've used LiveJournal for a while now. All of which, especially Facebook, has recently given me a bit of additional perspective on something.
You know all that stuff that I wrote about what makes social networking sites sticky? Sure, it's accurate as far as it goes, but I missed a big bit of it: time. Namely, the sheer amount of time it takes to keep up on each new site.
If people are actively using a social networking site, they're generating new content; tweets, LiveJournal entries, Facebook status updates, you name it. You have to keep up with all of this, and the more social networking sites the more time you get to spend on it; sooner or later you run out of time. The more time a social networking site can get you to spend on it, the less time you have to be anywhere else and the stickier it is.
(From this perspective, all those peculiar Facebook applications strike me as diabolically clever; they are a giant flytrap, trying to get you to spend more of your time on Facebook instead of somewhere else.)
So why does the number of social networking sites matter? One would think that all that matters is the total amount of updates, regardless of how many websites they're distributed across, and the total amount of updates would be more or less constant, since people only have so much free time to generate them.
I have two theories about this issue. The first is that it doesn't entirely matter, but that a site that you're already invested in has an easier time persuading you to spend more time there. The second is that there is a natural limit to the amount of updates that a social circle will tolerate on any single website, for reasons similar to how planet aggregators have a natural size limit; too many updates roll over people's front pages too often. Generate too many updates and you will get social pressure to cut back. But new websites create new front pages (and possibly different social circles too), and thus let the very active content generators cut loose again.