France and KDE are probably the two most well known adopters rn last time I heard. https://matrix.org explains the protocol, and if you join the MatrixHQ room (#matrix:matrix.org) you can ask all you want, lots of people around to answer, and even Matthew (project lead) is often on there too.
Adoption is going pretty well - from matrix.org we see around 10M mxids out there, of which around 5M are on the matrix.org server. Many of these on the matrix.org server are bridged from other systems (Slack, IRC, etc), but 3.5M are native to Matrix. It's hard to guess how big the wider network is because there are many folks who don't phone home or federate actively with matrix.org.