Chuqui has an excellent point about all of the tools we use:
Social networks only succeed when the value they return a value that is worth the investment -- in this case, the primary investment is time. What the value returned is depends, everyone will define that differently, but there has to be a commonality here that this investment is worthwhile. This lack of return is likely one reason why so many blogs get abandoned; after the initial rush of enthusiasm wears off, people realize that it's, well, work, and takes time and energy (and for many folks, for little return). So they drop out.
It comes down to one core issue: time management...Because the bottom line is, there are only so many hours in a day, so many days in a week, and if you don't budget them, you'll come out the other end wondering why you were so busy and didn't accomplish a damn thing...
The reason why I've maintained this blog for almost a year (has it really been that long?!?!?) is because the return on this particular investment has been significantly greater than the time and effort put in.
The reason why I abandoned Twitter and Ning is because the return on investment wasn't high enough, fast enough.
Facebook is starting to show some promise as a possible way to put all of my stuff together, but I just haven't had the time yet to investigate this further. What's kept me linked has been people engaging me in this environment in ways that didn't happen in the others.
Considerations of effort vs. return will be critical with ANY tool we ask our end-users to adopt. Will the return on investment be high enough and happen fast enough for it to stay in the toolkit once the patina of novelty wears off? Or will it wind up in the junkyard with everything else?
BTW - you should also read Chuqui's follow-up post on this issue. Thankfully, I may never have the problem these folks do....