Where does the income of a "small" site like Bitly comes from when there are no ads or whatsoever involved in their operation?
Typically a company like bit.ly works hard to build users and people dedicated to their service and then finds opportunities to sell add-ons or supplemental services such as: http://www.bitlyenterprise.com/pricing.html in which they make money from companies and enterprise clients using their data, or using bit.ly services to run their own URL shortener.
I wouldn't necessarily say getting into the URL shortening business would be wise right now. But, if you have a unique angle maybe. Here is a decent post on writing a URL shortener.