Hi there,
yes, your understanding that SS is the best selling agency is right on the money. They indeed are.
And these are my stats :
- contributor since around September 2009
- started with 10 images - growing port as time went by
- port size at the moment - around 590 images (not counting the vector - JPEG duplicates)
- payout set at 100 dollars monthly
- making at least payout almost every month for the past 1 year
- September 2012 (this month), total earnings : 210 dollars. It was a good month, but not my BME.

One thing to keep in mind : I am not a photographer, I am not an illustrator and I am not a graphic designer. I work for a small printing company and I know how to use Photoshop and Illustrator That's about all I know
One other thing for your info : I sell vector files a lot more than photos. That's mostly where my payouts come from.
Good luck with SS and give yourself a bit of time. You're doing great so far and it will only get better
Best,