If you're on Facebook it's super-easy now with the new "Post to Facebook" button.

(If you are not currently logged in you will be asked to login to Facebook first).
The colored in picture goes into a special photo album in your account, and is posted to your wall along with your comment.
It's a really fun way to share your creative efforts.

p.s. There's also going to be a new online gallery coming soon, where you can show off your artwork for the world to see!