There are a few issues hidden in your question. I'm first going to write about whether these postings create prior art. Next, I'll write about whether a suggestion from a third party alters inventorship.
Prior art and online postings: Public discussions create create prior art. If done in a public online written forum then the postings will count as "printed publications." They would also likely qualify as proof that the invention (or a portion of the invention) was known or used by others. Public responses to your inquiry would also be seen as a printed publication. However, a private message would not normally create prior art because it is not public. More here.
The prior art impact of of the online posting will depend upon when it was publicly posted relative to the effective filing date of the patent in question. There are three primary situations:
- Posted after filing the patent application: Not Prior Art
- Posted 1-year or less before filing the patent application: Prior Art in Some Cases
- Posted more than 1-year before filing the patent application: Definitely Prior Art
The gold standard here is to wait until after you have "secured a filing date" by filing a patent application. At that point you can communicate as much as you want without jeopardizing the novelty of your invention. Now, there are still reasons not to do this -- especially if you are planning to market your own product or if you are planning on filing further patent applications.
The statute that controls in the US is 35 U.S.C. 102. This provision was extensively revised in the 2011 America Invents Act (AIA). The old rules apply to patent applications filed before March 16, 2016 and the new rules apply to patent applications filed on or after that date.
The 1-year before filing uncertainty is based upon what we call the "Grace Period" for filing patent applications. The US is the only major country with a standard grace period that allows for pre-filing disclosures by the inventors. Thus, if you are seeking a European patent, any pre-filing online public posting could be used to block your later-filed patent from issuing. The US grace period allows patent applicants to make a public disclosure and then wait for up to one year to file their patent applications. This grace period was narrowed in the AIA, but its exact scope is a matter of heated debate between patent attorneys.
Inventorship: In the US, patents must be filed by (or in the name of) the inventors of the invention being patented. Often there is only 1 inventor. But more often than not, patent applications list multiple "joint-inventors" who each contributed to at least one of the patent claims. Under the statute, “Inventors may apply for a patent jointly even though (1) they did not physically work together or at the same time, (2) each did not make the same type or amount of contribution, or (3) each did not make a contribution to the subject matter of every claim of the patent.” 35 U.S.C. 116. The online suggestion would certainly be sufficient to create joint inventorship if the suggestion becomes part of your conception of the invention. However, the tricky part is that the the contribution will count as a joint inventorship if it was "simply a reduction to practice of the sole inventor’s broader concept.” Ethicon Inc. v. United States Surgical Corp., 135 F.3d 1456, 1460-63, 45 USPQ2d 1545, 1548-1551 (Fed. Cir. 1998).
If there are two inventors, each inventor has full rights to the patent. As you can imagine this could lead to some conflicts.