Q:

Why does the "people reached" counter use a lowercase "m" for millions? 

As far as I know, the usual unit prefix for millions is mega, written as M. I think it should apply to unit-less numbers as well. 
Thus, I suggest that
~4.4m
is changed to
~4.4M
It's arguably much cooler to see a bigger M anyway. 

A:

Forget the abbreviation; let's just use E notation...

4.4E6

Alternatively:

4.4×106

A:

This is pretty much the most valid comment:

Meta makes a good point: these aren't SI units, it's EN-US slang (more or less).  For most people in EN-* locales, the UX is better than SI units would be, with no loss of accuracy or real problems. 

We're not speaking of internationalized units here. 

As far as I know, the usual unit prefix for millions is mega, written as M.

The usual prefix for mega is M. It just so happens that mega is of the order of magnitude of millions, so a sloppy equivalence (similar to PHP's ==) makes people think that million actually means mega. 
We're not speaking mega-people-reached, or kilo-people-reached here.  Those unit signs should apply when referencing standard, internationalized units, not people who may have viewed your activity on Stack Overflow. 

A:

How about a compromise:
Use small caps for the M, like this:
~4.4ᴍ

