Just looking at commit logs, fs/xfs has had 830 commits since 2.6.32; fs/ext4 & fs/jbd2 have had 826. They are both very actively developed.
fs/jfs has had 114, for reference.
Commits are a coarse measure, but it should give you some idea of the activity going on.
XFS has never been as "main street" as extN, but that's a bit of a circular argument; fewer people use xfs because it's less familiar because fewer people use xfs because ...
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