As long as the people in the relationship are fine with it I'd say it's alright.
I'm usually a notorious flirt with my friends online myself actually, but out of respect for my boyfriend I stopped. I don't know if he'd be alright with it, and even if he was I wouldn't want to make him feel inferior somehow.
But flirting with him is just as much fun anyway. :3
I especially love saying things that make him blush...it's adorable.