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.
For your question, this is why I wrote the 1.2 solution there:
1.2. Using quote block without exactly original sourceUsing this method when you don't remember exactly where is the source of the content you post, or you simple write it down from your memory but you doubt that you might plagiarise somehow.
"Plagiarism is the "wrongful appropriation" and "stealing and publication" of another author's "language, thoughts, ideas, or expressions" and the representation of them as one's own original work.[1][2]"
[quote]
"Plagiarism is the "wrongful appropriation" and "stealing and publication" of another author's "language, thoughts, ideas, or expressions" and the representation of them as one's own original work.[1][2]"
[/quote]
For example, I can write this one, and don't plagiarise.
Facebook's founder is Mark Zuckerberg
[quote author=sources on facebook]
Facebook's founder is Mark Zuckerberg
[/quote]