Is it appropriate to acknowledge StackExchange in my MSc thesis?
Your acknowledgement is a place where you are free to thank anyone or anything (I have seen dogs being thanked) you think deserve mentioning. To acknowledge StackExchange is highly appropriate since it is a community driven open site where help can be obtained to solve problems from many different perspectives.

In my PhD thesis, I, perhaps a bit over-ambitiously, thanked Donald Knuth for creating TeX. Had StackExchange been in existence back then, some of the terror of writing my thesis in plain TeX would have been eased and it would have been placed along-side Knuth in the acknowledgement.