What does "MM" mean in this context?

Solution 1:

MM is an abbreviation for "million", short for "mille mille", or 1000 × 1000.

Encyclopaedia of pure mathematics, by Barlow, Peacock, et al., p. 426:

(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.

Solution 2:

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:

TechCrunch:

Von Ahn told me Duolingo now has about 25 million registered and 12.5 million active users.

Forbes:

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.