If you haven't had the conversation about the dangers of putting all your eggs in one basket, you should. The online writing market is fickle and sites can disappear without a trace in the blink of an eye. If all your work is on one site, or you depend on your income from them to survive as a freelance writer, you can find yourself struggling to meet your financial obligations if they make major changes.
But that's not the only reason you should diversify and publish your work on a variety of online writing sites. The Top Five Reasons Why Writers Should Publish on Multiple Sites explains why it's in your best interest as a writer to publish your work on multiple writing sites.
If you are only writing for fun and really aren't that concerned about making a living from your writing, diversifying may be of less concern for you. That's okay. Hobbyists sometimes prefer to stick to a site that is comfortable and doesn't require the added work of learning what works and what doesn't on a new site.
But, if you are serious about a successful writing career and want to create a steady income base from your writing, publishing on multiple sites improves your chances of success and allows you to develop a plan for putting your writing to work for you.