I'm currently writing my masters thesis and I've gotten some good answers that have helped me along in my process.
My question is do I cite mathstackexchange in my thesis or do I put a note of thanks?
There was a discussion on meta.mathoverflow about this, How do I cite Math Overflow?
I agree with what Mike Shulman suggested, which is putting a sentence in your acknowledgements:
Also, as pointed out by Noah Snyder on that meta post, this paper does a reasonable job citing math overflow, and the main answerer, in footnote 8.
I too have this issue coming up. One part of my thesis began on this site and another on MathOverflow.
My intention is to put a footnote link to the threads, give credit when I use someone else's proof which appeared on these sites and of course give thanks both to the communities and the users (by real name if possible, by user name otherwise) - both those who asked the question; gave helpful comments and so on.
According to the terms of service, paragraph 3, subscriber content:
To summarize, unless you created the content, you must clearly link to the post, link to author, and identify it comes from XX.SE site. This may seem like a pain but it can be a benefit if SE every has a math careers or academic careers associated with the sites that are academic in nature such as chemistry, biology, math, etc. The linked research content could then be added to userX's careers profile which in turn could help them land a first career or a new career.
Additionally, in the other answers, it mentions real names and only usernames if that is all they have; however, this practice would not follow SE subscriber content. You can put their real name if you know it but you must link to their profile with their user name.
Here is the screenshot from the accepted answer of the paper that does a "reasonable" job.
This would not be the correct way to cite subscriber content obtained from SE.
Here is how I cited the use of a LaTeX diagram: