The term does refer to actual catfish though, in the documentary that coined the term, "Vince, relays a story of how when live cod were shipped to Asia from North America, the fish's inactivity in their tanks resulted in only mushy flesh reaching the destination. However, fishermen found that putting catfish in the tanks with the cod kept them active, and thus ensured the quality of the fish. Vince made an analogy of how there are people in everyone's lives who keep each other active, always on their toes and always thinking, suggesting that people should always be alert while socializing through the Internet."
I don't know how some 40 year old pretending to be 18/f/CA keeps you always thinking, but there you go.