Does anyone have any ballpark figures for which IDE has the most market share amongst developers?
I personally don't know any reliable marketshare stats.
Most people I trust in that domain (mostly because they are on the field and see diverse teams in real action and not only evangelists in conferences) say that it's now about half-half Eclipse and IntelliJ and a very small fraction of NetBeans for Java developers. If you get to other domains, you see different results including some areas/personas where Eclipse IDE is almost non-existant despite some offering on Marketplace. The rise of VSCode also impacts this trend deeply; not so much in the Java world yet, but in the Front End development world and others, it's rapidly taking a big share (and not particularly an Eclipse share, other IDEs/tools are also affected for some of them in much stronger ways). But we don't see too much impact on Eclipse user downloads (see last paragraph).
There are multiple surveys of various quality you can find in the Java/developer ecosystem and some forums, but they are not to be trusted blindly and require some extra analysis to decide how much trust you can put on them (ie how their participant are representative of your target audience).
There is also http://pypl.github.io/IDE.html
which is a bit better as it captures data which is not benevolent participation (capturing all search requests from everyone), but the way it measures things is error-prone as it can capture a lot of false-positive.
However, the Eclipse IDE downloads stats are public (you can see them on
the download page). If you're an Eclipse commiter, you have access to the finer grain download stats. I didn't check them for a while, but for the month of October, the Eclipse IDE packages and installers were downloaded ~1 million times (this doesn't count "on-site" updates, only downloads of the whole stuff). We do not have such download stats for other IDEs to compare.
We are trying to look at it regularly and start to establish a trend, but really 1 million downloads monthly is a good number and seems consistent with the 50/50 approximation of IJ vs Eclipse IDE.