my 2 cents:
What stage are you at in a relationship?
If you are in the "window shopping" phase of things, looking for a potential partner, chatting with or messaging various people without disclosing this information to all your other contacts, nope, not cheating. My understanding is this is what would be expected. Nobody that I know of goes on these sites, picks one person and communicates with that one person solely.
Having finished your window shopping, you have decided to try out a few. Kick the tires.

Maybe (if you're lucky) actually meet a few in person. You try a few, some work, some don't. Is this cheating? Nope.
To simplify the stages, I'll boil it down to this. Eventually you develop an emotional attachment to a person, commit to that one person. Yet, you continue on with chatting or messaging with members of the opposite sex (or whatever sexual orientation you may be).
Cheating? Yes... and no. It depends on the situation.
When one enters into an exclusive relationship with another, there should develop what I'd like to call an "expectation of personal boundaries" within a relationship.
Here's where communication plays a key factor. You are now a couple. any friend you have of the opposite sex should now be "our" friend, and vice-versa. If you keep the fact that you are still on these sites a secret from your now significant other, or he/she does the same....
why?In a nutshell, when in an exclusive relationship, honesty and disclosure = not cheating. What's the opposite of honesty and disclosure? Oh yeah.. lying and hiding.

Cheating? If you ask me,
damn skippy! Sorry if that sounds a little black and white with no room for gray areas, but that's just my opinion.
Does finding out that your significant other still uses these sites present an insurmountable obstacle to your relationship? With proper and effective communication, probably not.
I'll shut up now.