Few things on the Internet bug me more, as an editor, an artist, and a human being who loves language enough to want to see it used properly, than the constant throwing around of accusations of plagiarism by people who don't seem to have taken the time to learn what that word actually means.
Plagiarism is when someone takes someone else’s original work and claims it as their own, either intact or after making only very minor alterations that leave it still recognizable as the original work. For instance, if I were to post this
“Faith is the downy thing
That lands upon the heart
And dances a dance without a beat
Missing time apart”
as an original work by Harlean Carpenter, that would be plagiarism because, for any of you not familiar with it, this
“Hope is the thing with feathers
That perches in the soul
And sings the tune without the words
And never stops at all”
is by Emily Dickinson. Anyone familiar with the original would see immediately that I had ripped her off, blatantly. And badly, but give me a break, that’s just something I dashed off to prove a point.
The following two things, the most common of the completely false accusations I see, are not plagiarism:
Someone buying a copy of your book and then selling it is not plagiarism, it is reselling. If they took a marker, blacked out your name, wrote their own in its place and then sold it, that would be plagiarism. And stupid, that would also be really really stupid. But just selling something they own is not a crime.
Someone sharing your Facebook/Myspace/Twitter posts is also not plagiarism, and frankly it blows my mind that anyone would think it is, but apparently a lot of people do think just that. So, to clarify for those people... no. If you post something in a public forum, and someone shares it in the same public forum with your name still on it, that is not any kind of plagiarism. I'm sorry if you don’t like the opinion of your comment/status/tweet they may have added upon sharing it, but that does not make them plagiarists. It might make them assholes, but last time I checked that's still legal in most states.