I'd say that there are no universally applicable standards here, given that it's just a place for people to talk. However, the more rigorous the standards you use, the more seriously people will take you. If, for example, you make some wild comment without anything to back it up, that's your prerogative, but don't expect people to believe you. If you want to make a case, give evidence.
The problem with the case you mention is that no-one would actually know that plagiarism had occurred unless, like you, they'd happened to have read the book being plagiarised. It seems to me that the poster in question is actually shooting himself in the foot, to some degree, given that citing the book in question would presumably reinforce his argument. It would, at least, mean he could throw a source at those who disagree with him, rather than rely simply upon assertion. So this sort of plagiarism isn't in the same league as someone who plagiarises in published work, or in work done for a degree, or something like that: the poster isn't benefiting materially, any more than if one were to plagiarise in conversation (if such a thing is possible). But it's a very odd thing to do and strikes me as slightly seedy, if only because the poster gains informal credit (at least on these fora) for ideas and words which are not his own.
So at the end of that ramble, I'm still going to vote for the second option, as it's where my instincts point.
)...
