She didn't feel like hanging out with you two the other night, and didn't want to hurt your feelings so she fibbed about being tired. I've done it myself, and I suspect most people have. Sure, it's a lie, a white one. I don't think this necessarily indicates that she would be a liar about things that are actually important. At worst, she's a bit of a flake. I am assuming you hadn't already cooked her a four-course dinner or bought event tickets for the evening, and her canceling didn't mean you spent the evening alone, you had your husband with you.
While I personally would prefer dental surgery, without Novocaine, over dating a committed couple as a potential "third," I sure would run far, far away from ANY potential partner who was stalking me on Facebook and wanting me to account for my time.