You can become successful at it but it takes hard work and a lot of dedication like anything in life. I've looked at some photographer profiles on fotolia, and I saw one guy who uploaded only 2000 images, but made 40K in sales. Normally it is not possible to get so much sales on this amount of photos. Stock industry has to be understood, and it is always an experiment because times change, events happen, styles evolve. You never know for sure which image nails it. If you plan to photograph people, then you have to understand that clothes, and hair styles change, eventually as the style changes, people become less hip, tech such as laptops and phones start to look old and therefor such images become non-sellers. On one hand it is bad, on the other hand that means you can reshoot your best seller subjects to be on the cutting edge with the times. This also means that you are never really late to the game.
To make money in stock you have to sell on more than one stock agency. Best stock sites are istock, dreamstime, fotolia, and shutterstock. Sometimes one image gets accepted on one stock site, but not the other. Sometimes the same image will be a big hit on one stock site, and a non-seller on the other. If you have access to special places or subjects, thats a good way to gain traction in microstock. I started by having access to a dental laboratory that allowed me to shoot some of their prosthetic products.
I sell only a little bit on stock sites, usually stuff that I snap somewhere that only sits on my HD doing absolutely nothing for me. Uploading stuff like that on microstock pays for my daily Americano at Starbucks. One thing I've learned is that these stock agencies always tinker with their search algorithms. I had some images, that always came up on first page with particular search terms, and thus made me good sales, but suddenly fall into abyss (I mean page 300), and the sales went from hero to zero in one day. It's like google, being on first page matters big time. A crappy image on first page will outsell your award winning image on 10th page every single time.
Always pay attention to keywording. Properly keywording an image is crucial to success. Some stock site will allow you to modify keywords after submission, and some will not. I do all the keywording in Lightroom so they will be embedded into the image, that way I do not need to keyword each time on every stock site.