Is it correct word usage to say that I'm mobbed by a single person? The word mobbing comes from mob, which refers to a large number of people, so only one person mobbing sounds a bit strange.
It depends on the usage. Perhaps I woudn't say:
I was mobbed by Bill.
but that doesn't mean I couldn't say:
I felt mobbed by Bill.
if Bill's attention was so overwhelming that it felt like I was fighting off an entire group.
I have heard it as well, however it is slang usage to portray that you are being hit with an onslaught of attention by one person.
Dictionary.com does not include reference to a single person anywhere in its definition.
You are correct.
Saying "mobbed by one person" does not apply in formal language.
Informally though, it is used very often (gladly not where I live).
I would compare it to saying "a couple of hours" or "a couple of minutes" that should mean "two hours" or "two minutes" respectively, but it's often used when referring to "a few hours" or "a few minutes".
Incorrect, but often used.