How do URL shortener services make money?
I'm really curious, so they are able to live off donations (if any) alone ?
In other words, I'm interested what will the business model of these companies look like?
Bit.ly makes money through its enterprise service, offering custom shortened URLs (using its technology to manage shortened URLs through a branded domain, like pep.si) and advanced analytics.
Other sites are making money simply through advertising on their main site. TinyURL gets ~3,000,000 monthly unique visitors, which translates into an even higher number of page views -- the banner ads displayed on the site likely provide a good amount of revenue. URLtea.me looks like it has paid links on the front page -- because so many people are linking to the URLtea.me domain, those links likely pass a high SEO value.
Blue.gg seems to be a promotional effort / "cool factor" property for a digital creative agency. I imagine a substantial number of less popular URL shorteners are run by people simply for the cool factor or convenience of personalized, super-short links. Because these services are only redirecting people rather than serving up content of their own, the bandwidth used is really very low -- many services aren't being created to make money, because they don't cost that much money to run.