Can a Buddhist monk use social media for his teaching? As far as I understand, there are sets of rules to be observed as a monastic monk and I'm not sure if TV, radio and other source of media is allowed?

Just curious to know, no offense.
A monk can appear in all social media, but there are, according to the Navakovada, some things he cannot talk about, if certain conditions are not met. Since he does not know whom is out there doing what, he might choose not to teach the Dhamma.

This is a bit of a grayzone, so it is up to each monks own evaluation whether he will do it or not.