Can I be lawfully terminated for an inappropriate facebook comment?

[john]

I made an inappropriate comment on my personal facebook page about my boss on company time and on a company laptop computer. My page is private and I only accept people that I know personally and I'm friends with. How my boss found out was he had a keyword logger / screen capture type of software on the computer and was able to see my page.

Now I know what I said was wrong, but my question is this legal? I never signed any type of document stating that this type of software was on my company laptop. And is it legal he was able to see my facebook page with the screen capture software when my account is private? If it helps I was employed in the state of Maryland
Also, in the facebook comment I never mentioned my boss's name.