To answer OP:
I work for a UK University. I would describe us as a medium size enterprise in computing resource terms. For many years we have run a Solaris system, and it was excellent, very reliable. However, the Solaris system is just about dead and I think will disappear completely in the next couple of years. There are several reasons for this, and I suspect they will apply in lots of other "medium size" companies.
The first is that the actual decisions are taken by what someone higher up this thread called "morons". I think that is rather kind, but I will stick with it. The morons don't understand things, so when companies like Microsoft come along and say "outsource your email to us and it will be better and cheaper because you won't need dedicated technical support" the morons think "great" and do it. The result of that one has been that our email service is now virtually unusable, with many of my colleagues setting up redirects to Gmail accounts. It's difficult to go backwards though because the technical support staff have now gone. We have bought into things like Google Docs and Alfresco Hub, both of which I hate, and both of which seem to be much more Windows (or maybe Linux) oriented than OS agnostic.
The second is that Oracle have really killed Solaris for organisations like ours. We used to get a reasonable discount as an educational institution. Oracle stopped that and also put up all the prices, so the difference for us was huge. The result is that we have dying servers no longer under support contracts, quite a big network of Sunray labs running an absolutely horrible Ubuntu desktop, and again tech support staff not being replaced if they leave.
There are other factors as well (the users don't help!) but they are probably the main ones.
The result of all of this is that we have labs running Windows on PCs that are rubbish in terms of reliability compared to the SPARC-based server systems we used to have. They are much less reliable to use and are on a 3-year rolling replacement cycle and overall probably cost far more than retaining Solaris might have done, but not in a way the morons can understand.
The other thing we have is Linux. There are lots of reasons why, but the two main ones are probably (a) the morons have at least heard of it, and (b) there are people around who feel they are competent to support it (I think many actually are not, but that's another story). None of these people have even heard of FreeBSD as far as I know. I have FreeBSD running on my laptop and, when nobody was looking, I replaced the approved Windows image on my desktop PC with FreeBSD - nobody has noticed and I haven't got into trouble yet. So, there are two FreeBSD systems where I work, out of several thousand machines. If there are any others, I have not encountered them.
As others have said, learning FreeBSD will stand you in good stead for finding your way around any other Unix system, but I think admin jobs specifically requiring any of the *BSDs will be relatively rare.
I'll end with a bit of prejudice: the downside to knowing your way around any other Unix system is that in many ways, Linux is not a Unix system. It's something that, from a user perspective at the command line, looks like Unix. But under the hood, as they say - wow, what a mess.