Dreamstime is one of my favorite Microstock Agencies. Their website takes a little longer to get used to and tend to refuse all vector illustrations they think are too simple, but you'll regularly sell your images here and often Dreamstime Portfolios have the best Google Ranking.
Why Shutterstock? Shutterstock is, without any question, the number 1 best selling agency for most independent microstock photographers and illustrators. I personally earn about 50% thanks to their website. The company was founded in 2003 by Oringer, who uploaded 30,000 of his own stock photos and made them available via subscription, forming the basis of the company.
Founded in 2004, the agency mostly sells to Europe, in particular to Germany. The Website is of course international, Fotolia is active in 23 countries and available in 14 languages. The agency was bought by Adobe in 2015. This means that you can reach millions of Creative Cloud users and sell directly through Photoshop CC, Illustrator CC, InDesign CC and other desktop apps. Being part of the huge Adobe Stock network should soon start to show an overall raise in sales.
This process can be frustrating sometimes and you may not always agree with the reasons that are presented to you, but keep in mind, that your portfolio looks a lot better without unnecessary files that don't sell. The moderation helped me personally, to improve the quality of my own portfolio over time! Stay patient, read the reasons for rejection and avoid them the next time.
The Microstock Guide 