I'm a sysadmin / network admin who has traditionally made my living in a windows environment. At the size of companies I worked for LInux networking and system monitoring tools just made way more sense then anything in the MS ecosystem. I started learning Linux because I like to learn and I can deploy it to test without cumbersome licensing issues. I also dabble in system forensics and Linux is an obvious choice for your toolkit.
WIthin the past couple years I've transition to mostly LInux system administration. I find there are less jobs out there, but the wheat to chaff ratio is much better. Employers respect and pay Linux Admins better, and the average Linux Admin is, IMHO, more likely to have the love of learning and curiosity that differentiate between the good and the great.
I still get dragged into the windows world, because I understand both and I have alot of citrix experience, which translates into a pretty good understanding of AD and GPO.
I only see Linux growing, more companies seem to be embracing a mixed environment, even if they start with unpatched virtual appliances.