Once something is made publicly available it is, for all intents and purposes, published. It doesn't matter whether it's a blog, your FB page, display site, etc. (As Harlequin mentioned, the SYW forums here are password protected so Google bots can't see them so content post there is not considered published.)
When it comes to something like a novel, you will not likely do any harm in posting a chapter or two if you want. You can even post a few paragraphs or lines from later sections of the book and no one will blink about that (although I would be wary about posting anything that is spoilery). But the more you post for free, the less likely a publisher will be able to make money off it.
There are exceptions, of course. Some folks have self-pubbed/serialized things that turned out to be so popular a publisher was still able to provide it in print format and make money. But those exceptions are *not* the norm.
Also, something to consider is that you want folks to read your work and be excited by it and want to buy it. But, if you are sharing chapters too far ahead of publication, they may forget about your book when it finally is published. (As you build a fan base this problem becomes less of an issue, but if you are just starting out you primarily want to be pointing folks to stuff they can read right now. Not in 3-4 years. This is why the short stories in magazines model worked so well for folks - they could have work out fairly quickly, recommend it to folks, and still be working on/querying/etc a longer work.) Remember also that once you are working with a publisher they will likely want to release a chapter or two leading up to publication and that will have more impact if folks are seeing that content for the first time.
So, you don't want to make more than 2-3 chapters public (especially if they are long), and ideally you would likely be better off posting shorter teasers than that even.
Best of luck with your querying!