I think it is not so much the size of the world, as a problem of time. This is a full-time universe, but it is inhabited by part-time beings. Every one of us comes here burdened with a dependency, on that place we call real life.
When people first become involved with SL, it's common that we'll forgo sleep, skip meals, anything just to spend a few more hours building things, and playing around with friends. It makes for a joyous escape. But after a while RL and its demands creep insidiously back into our time.
There comes a day when you decide you'd rather get a couple hours of sleep at night. Or that you'd like to get the kitchen clean or the bills paid before you sign on.
Or you decide to head to a movie with some RL friends. Or meet your coworkers for happy hour.
Pretty soon you find you're only spending a couple hours a night in SL. And so are your friends. The trouble is, they live on the other coast, or on another continent, and it's not the same couple of hours that you are on.
After a while you no longer are able to coordinate a common activity together. All contact comes through saved IM's. Ships passing in the night. Then you sign on to find that they have found other folks to spend time with, since you are "never on". Or you check to see who's around, but no one is, so you sign off. Then they check half an hour later, realize you aren't around to brighten their day, so they sign off.
Eventually running into each other becomes something that happens only every few weeks. At that point even IM's seem to get lost in the dust. And so it goes. The world seems a lonely place.
RL wins.
