I assume it would be sufficient to write something along the lines, 'I read on facebook' or 'I saw on TV' when writing a post to avoid being reported for plagiarism.
One might see an interesting discussion on the facebook wall without commenting or liking it. So that post doesn't become part of your activity log. The next time you open facebook, it is no longer there.

Or you might remember a discussion you saw on TV without knowing the show, person etc. Now imagine that you write a post mentioning what you heard on TV or read on facebook and it turns out that a very similar rephrased or exact discussion already took place somewhere else.

If the user doing the reporting or the admin who checks the report doesn't pay enough attention he might think you stole the information from that source without providing a reference while in fact that wasn't the case.     

So my question is, would it be enough to write 'I read on facebook' or 'I saw on TV'? In theory it should because you are not claiming it as your own, you just can't provide a source link to something you don't know is there.