To use StumbleUpon effectively you can add your site to StumbleUpon and kick start your submission with some paid stumblers and if people like your page then after the campaign stops your page would still be in the stumbleupon directory and you would continue to get traffic from stumbleupon.
This is a good point! As after this you are not doing anything to promote it. Even if the bounce rate is high, even if 2% of people become your member without any pay or anything, then whats the harm
Nice info..