I know. It still doesn't make sense when people use k/M for 1,000, M/m/kk/MM/mm for 1,000,000 and B/MM/mm for 1,000,000,000. (sorted by ~popularity)

It's even more annoying when someone uses "k" and "mm" in the same post, since the whole idea of "mm" comes from repeating roman "M" (1,000). So why not stick to k/kk (WoW-style) or M/MM (almost-roman style)?

At least "k" and "M" are based on some kind of standard.

From the link: "It is commonly abbreviated as m or M; further MM [...], mm, or mn in financial contexts." Note the semicolon.

Also note that the only cited source additionally claims that "M" is often used to indicate one thousand, as in $60,000 = $60M, which for all I know may be true in the financial world but is definitely not true for anyone outside that world who wants to be understood.

Is it really? Your link says:

  It is commonly abbreviated as [...] mm, or mn in financial contexts.[5][better source needed]

It can't be an abbreviation as that refers to subtractions of letters existing in the word and I only count one 'm' in millions.

Yes it's pedantry and yes I understand that 'mm' is in someways a convention but it's a terrible one.

As long as we're being pedantic, abbreviation does not refer strictly to "subtractions of letters existing in the word," it is any shortened form of a word or phrase.

https://en.wikipedia.org/wiki/Abbreviation (see especially the section on plural forms)

http://www.oxforddictionaries.com/us/definition/american_eng...

http://www.merriam-webster.com/dictionary/abbreviation

How is adding non-existent letters any form of abbreviation?

That's all I was stating.

Because it is a shorter form of the word it abbreviates. 'mm' is shorter than 'millions'.