[SauronOfMordor]

I know of no culture or place where the use of M would imply a million.

In the US, particularly among people who use computers, "M" is used for million (mega) and K is used for thousand (kilo)

[crystalk]

I myself sometimes use the LOWER case m to mean million, where that would be clear, but the upper case M is always a thousand AFAIK.

Of course it is wise to use these only where the context makes the sense clear, I would have thought that to be the case in the quote about weekly US Muslim worship attendance, obviously that could not be in the Millions, much less the hundreds of Millions!