Yes, it's certainly possible that posting on the internet could lead to someone stealing your ideas. 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. Does the site do a good job at giving feedback? Knowing that will help you decide if the benefit to you is worth the uncertainty and unease it might cause to post draft material publicly.
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.)
You should also read the user agreements of where you post. This very site used to allow critiques, and there were issues associated with it. I'm not a lawyer, but this is how I understand the problem: When you post on a Stack Exchange site, you have agreed that what your words on the Stack Exchange site will become a Creative Commons work. I think that this includes the excerpt you post - if so, it can be distributed without further permission from you.
I don't think this means you give up your copyright to the excerpt, and perhaps the previous copyright on the words means you're just giving permission to use it and not converting the excerpt into CC material? I'm particularly unclear on this part. This issue has been discussed, but and I haven't seen a definitive answer to this problem.
Are critiques allowed on this site?
No. You can use your own work as a way of illustrating a specific, answerable question, but you can't post work for general critique. Critiques used to be on-topic here but it didn't work out well; the community has since decided that they're off-topic.
You'd probably be fine posting work on the web for critique, but there's no certainty. You need to decide for yourself if the feedback you might get is important enough to you.