The question isn’t “how do you see yourself online?” but “How to others see you online?”
Allowing for the vagaries of memory, selective unconscious editing, self-justification, etc., I quite confident that how others see us online isn’t the same thing as how we see ourselves.
The saying “know thyself” is often repeated and for practical purposes, is about as effective as a poke with a sharp stick. It hurts but there’s not much other benefit to be had.
Farhad Manjoo writes in ThinkUp Helps the Social Network User See the Online Self about the startup Thinkup.com, which offers an analytical service of your participation in social networks.
Unlike your “selective” memory, Thinkup gives you a report based on all your tweets, posts, etc., and breaks them down in ways you probably would not anticipate. The service creates enough distance between you and the report that you get a glimpse of yourself as others may be seeing you.
Beyond whatever value self-knowledge has for you, Thinkup, as Farhad learns from experience, can make you a more effective user of social media. You are already spending time on social media, why not spend it more effectively?