But IMO, it could be a case-to-case basis and if you can see the (accused being plagiarized) post of Mpamaegbu, the intention could be possible is to show a reference or content that come from the online article (or somewhere else) but the big mistake is, Mpamaegbu forgot to include the link or must be quoted it from the original author, which is this is very strong evidence by suspecting of being a plagiarist. So IMO, that could be a mistake by him by not being accredited to the original author.
There are a lot of people who basically copy entire articles in a thread and have a citation at the bottom of the post. This is not plagiarism, but IMO is another problem as the post has virtually no original content nor commentary. The above is often not even someone summarizing an article, it is copying entire articles.