I quit uploading to microstock about two years ago after being in it for six years. It was fun and fairly profitable but I decided to spend my time on my landscape and other fine art work. What most photographers don't understand is that the relatively low commission rate (.25 to $2.00 on average per sale) actually turns out to be really good on a per hour of work basis.
I've spent zero time on microstock over the last two years but but I'm still making several hundreds of dollars per month from an average of about 700 images across five sites. I haven't seen much reduction in volume sold either. Whatever key words I used long ago are still working rather well. So I'm just sitting here collecting money every month. I have no idea how much longer this income stream will continue with no new input. But I'm lazy enough to want to find out.
The key to microstock is to understand that the low commission rate, over time, converts to quit a tidy sum over time.