It’s a really bad habit of certain financial types and inappropriate almost anywhere. Hacker News is not a financial hangout and the story is not even primarily about finance. Even a financially oriented news service like Bloomberg usually writes “million”:

http://www.google.com/search?q=site:bloomberg.com+intitle:mi...

M is the Roman numeral for one thousand. In the financial press, MM is M*M or one million. I've also seen it used with BTUs.

Some cultures (UK?) actually do/did use M in the Roman sense to mean thousand, so 33M would mean 33 thousand. That conflicts with metric Mega 10e6, so K is becoming more popular.

In Rome of course, MM would be two thousand.

In short, it's complicated. Or at least context-sensitive.

It's fairly common in financial circles and it means "million", ie 10^6. In the same vein, MMM can mean 10^9.