>> It’s informal yes, but this isn’t a place for formal writing.

>> So it’s mostly just you.

It's more likely a generational thing, so not just him/her. It's also less about formal writing than it is with familiarity.

My (older) generation typically doesn't refer to someone by their first name only unless we actually know the person.

That is precisely why those who want to place themselves in the in crowd use first names.

Perhaps it once was but it's a really common thing now so it's hardly just for that reason.

Or perhaps narcissism and attention-seeking behaviours are "a really common thing now" and, as a consequence, referring to wealthy strangers by first name is also becoming more common.

No offense, but this comes across very "old man yells at cloud" or "darn kids" and I'm 37.

Common among a particular age group or demographic doesn't necessarily mean it's a universal behavior.