Nicely done, website is nice and it's great that you can just click and get started straight away. In your comment you said you moved away from mySQL, what exactly do you store in the DB? Which leads to the question: how do you transfer the drawing data between clients? I guess it's not a case of uploading a Canvas as an image?
The drawing is basically a list of strokes, and a stroke is a list of points - that is, (x, y) coordinates. So the database is mostly a huge table of (x,y) coordinates.
Uploading the entire canvas would far too inefficient - I simply upload the points.
The transfer between two clients uses a technique called 'long polling'. That is, the client sends an HTTP request to the server, and the server doesn't respond until it has received a message (ie. stroke) from another client.
paper.js has a nice method to do this. I actually used it in a whiteboard app I made as a proof of concept. Rather than saving every X,Y coordinate, it uses a configurable number of points and handles and makes bezier curves. http://paperjs.org/examples/path-simplification/
