Most interaction - Facebook, MySpace, MeetMe, dating sites, forums, blogging, even comments on news articles - these all provide feedback to the author. Narcissistic supply. He is being heard.
Just by answering you are creating a validating feedback loop. Whether a direct "like" or "you're right", or providing more information, you are validating the writer's effort. Hell, even trolling!
In a way it's saddening, that we've gone 100k+ years from cavemen grunting to the intellectual platform of the internet. Something that is actually used for putting likes and gushing comments on cat pictures and the vapid sayings of the rich and famous.
Or Gangnam Style dance videos.
Or this place...