When I started our software architecture course about eight years ago, I was happy to find out about a book series on the architecture of open source applications. I was thrilled: Not only code, but architecture descriptions! I expected great material for my course. Sadly, I had to realize that none of the chapters in this book series used UML, a staple of most academic software architecture courses.
Does this mean UML is not used in practice? Of course not. It is just not used in open source projects. This doesn’t imply that UML doesn’t have its uses. However, they may not be what tool vendors are telling you.