Quite important. For my current photography project (52 Faces) having exposure and gaining feedback through that is something that gives me motivated to keep making the photo's and to upload it every Tuesday. When I started the project I've told quite a lot of people and I know that many of my friends are eager for a new photo each week, it keeps me going although the project is not for the exposure but it helps.
As for the rest of my work, getting exposure is nice but not something I work for.
Well, I'm not sure what "exposure" particularly means. I don't care much for popularity but I do care about the opinions of the people who read my stuff. I'm a writer so my greatest interest is simply knowing what people think of my story and what they think I could do to make it better. I prefer comments to favs any day. I am actually a bit offended when someone favs without commenting because it seems to me as they would rather click on a fav button rather than let the author know how they feel about the story.
in fact, it makes me a bit uncomfortable. I submit my art partly to keep a history log for myself, and partly to show my friends what I've been up to. when strangers come running by the truckload I get uncomfortable.