I'm developing a twitter application. Twitter might or might not be a fad. I can't see so far into the future and while my prognosticating skills are decent, I'm not 100% certain. Hell, I thought friendster was hot.
Anyway by developing this one application, I am learning about social tools (which I've never done before), neural networks, semantic analysis, markov models and a hell of a lot of stats techniques I previously did not have a lot of usefulness to me. In the grander sense, I was more comfortable with deterministic techniques rather than probabilistic techniques.
If Twitter goes down for good, I lose some time but I take that new knowledge and make the next app that gets me closer to my 20 year goal.