Do you think your site is plagiarize, when you post 1 to 2 sentence of it on other sites like forums and blogs? Does posting your site contents can affect your site's content quality and affect your Page Rank?
the answer is no if you are posting a snippet of the content. on the other hand, if you or anyone else post the content then your site is not going to get affected and neither its PageRank
Yes, but it's not all black or white, for example quoting or posting little excerpts is totally fine. Especially on forums and blogs it's done millions of times daily, preferrably you should mention the source, but I wouldn't call this plagiarism.