There are lots of definitions for MM, but which one is used in the following two sentences?
25MM+ people (+14x Y/Y) use Duolingo app to learn new language
12MM+ teachers / students / parents (+15x Y/Y) use Remind101 to send 500MM+ messages
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MM is an abbreviation for "million", short for "mille mille", or 1000 × 1000.
(99.) It has been from a very early period the custom of writers on Arithmetic to separate numbers into periods of three and six, as the numeration in most European languages must proceed by thousands and millions; these periods are called membres by Stevinus, amongst whose definitions we find the following: …. Instead of million, he says, mille mille; for a thousand millions, he uses mille mille mille; and for a billion, mille mille mille mille, and so on for higher numbers.
MM stands for million in this case. Wikipedia says this about the word million:
It can be abbreviated MM in some financial contexts.
Your sentences come from this report from Kleiner Perkins Caufield & Byers, who is a financial firm, so it is understandable that they would use MM to refer to millions.
The numbers mentioned in that report are similar to other sources that discuss Duolingo and Remind101:
Von Ahn told me Duolingo now has about 25 million registered and 12.5 million active users.
Now 10 million people use Remind101 when you add its teacher base to the students and parents they work with, spreading fastest in the southern United States, sending 65 million messages per month.
MM here means "million". M is for "million" and it is doubled to indicate that it is plural. Of course 25MM+ means "more that 25 million."
Note that there is a dialect thing here. Generally speaking you would say "million" rather than "millions", and generally speaking you would just use one M, as in 25M+. However, in some dialects, especially some English dialects and especially in formal contexts, the plural is preferred and you would use MM, and sometimes even say "25 millions plus".
The doubling is used in a few different abbreviations in English. It definitely has a little bit of an old fashioned flavor to it, but in some contexts it is common.
For example, a lawyer might refer to a section of a contract as "section 10.1f" meaning that section and the following one, or they might refer to "section 10.1ff" meaning that section and several following sections.
Similarly in Biblical studies "vs" means "verse" -- as in verse of the Bible -- and "vss" means verses. Similarly when referring to manuscripts the abbreviation "MS" is common, meaning one manuscript, but "mss" is used to mean several manuscripts.