I believe flirting on the Internet is a form of cheating. I think it is a clear indication that there is some sort of problem in the relationship that needs to be addressed. While flirting on the Internet may not be as serious as having sex with another partner, it certainly seems the actions would lead to that scenario.
Yes, flirting on the internet is cheating, because it is being emotional with someone other than your spouse. Anytime a person tries to get attention from someone else when they are in a relationship, it is cheating. It is important to be honest to a significant other, and that includes activity on the internet.