No idea. I asked on twitter, here, and sent a few requests to online SU friends (like yourself!).
I don't ask for stumbles on everything but when something's really polished I try and give it a push. In a couple of days, after it's seeded, I'll have a look through who's stumbled it and send it to a few more friends that might also like it. Sometimes this second attempt brings in as much traffic as the first.
I think the important thing is to actually enjoy using SU and try to build up some real connections. Knowing what your friends might like to read is much more important than having 20,000 of them.