Yes, it's certainly possible that posting on the internet could lead to someone stealing your ideas, here or on any other site.
But will this actually happen? There are risks, however small, to showing your work to anyone. Most writers that publish know the benefits of peer feedback, and take the risk anyway.
Many people have reaped benefits from posting excerpts on the web for critique. On any site that you're considering posting, I'd suggest browsing the critique questions already up. Decide for yourself if the does a good job at giving feedback, then you can decide if the benefit to you is worth the risk. Or if it's worth the uncertainty and unease it might cause to post.
Peace of mind
Do you feel that it's likely that someone will steal your ideas? If posting stuff on the web for feedback will make you uneasy, you have the option of sticking to in-person writers groups, or forums that are password-protected, or even an email-only criticism group.
Framework and execution
There's a difference between book ideas and an excerpt you'd post for criticism.
The writing world is filled with people who come up with ideas for novels, or a grand epic series of novels. It's so common that we even have a name for it: world-builder's syndrome. The framework behind a novel is important, and it's vital that the structure of a story be interesting and sturdy. But the execution is what's truly unique. An excerpt of actual writing posted on the web is a piece of that execution. It's probably unlikely to be stolen, but I don't think anyone can prove this or even cite you vague numbers.
None of this is meant to say that people shouldn't post for critique here, but they should be aware of the possible issues:
Posting work on the web could, theoretically, cause problems with first-publication rights (what you would give a publisher). Small excerpts posted on the web are unlikely to be a problem, though. (Anything posted here would likely be an early draft in any case.)
And, closer to home: Posting on a Stack Exchange site automatically converts the what you post into a Creative Commons work - including the excerpt you post - which can be distributed without your permission. I don't think this means you give up your copyright to the excerpt, but I'm not a lawyer and can't speak to this. This issue has been discussed, but and I haven't seen a definitive answer to this problem.
Are critiques allowed on this site?
Yes, they are, and there are guidelines. But what they boil down to: A few paragraphs are optimal. A sentence or two will be closed immediately (we don't critique anything without context). Most importantly, you have to ask specific questions. There's more info on this in our critique guidelines.
Keep in mind that any site you post on may have their own requirements.
You'd probably be fine posting work on the web for critique, but there's no certainty. You need to decide that for yourself.