It's been a rough decade or so for Microsoft. It's lost its position as the most valuable tech company in the world to its old rival Apple, and even though Windows retains a huge market share in PCs, that market is collapsing, and Microsoft has struggled to keep up with Apple and Google when it comes to tablets, phones, and watches.
In the process, it's lost considerable ground among developers, as this chart by Joshua Kunst (via Ed Tufte) illustrates:
To some extent, this was a result of who was using Stack Overflow in the beginning. It was started by two veteran Windows developers, Joel Spolsky and Jeff Atwood, and the site itself was written using C# and ASP.NET, so its early users were disproportionately interested in those topics, and the subjects were bound to lose relative popularity as the site gained users outside the C#/.NET community.
But the chart also says something about Microsoft tools' declining relevance. C# was still in first place in 2011, when the site was already popular enough to be getting more than a million unique views a day, but then tumbled. ASP.NET and .NET fell even more dramatically, so much so that the latter didn't even make the top 30 tags in 2015.
By contrast, Java — long a major competitor to C# — stayed at the top largely because Google chose it as the main language for writing Android apps. Windows Mobile, where C# is usable, never took off, and Android is now by far the most popular mobile platform.
But Microsoft's decline isn't just about smartphones. Server-side web programming languages that compete with C# and ASP.NET — like PHP and Python — gained while ASP.NET faltered.