Great post! RunKeeper is indeed an excellent example of a great product, making marketing easier, as Stacy pointed out before. I would suggest that there are three steps needed to be successful in any business:
1) finding out what your customer want
2) building a great product
3) making sure the world knows about it.
(not necessarily in that order, and preferably in a continuously repeating cycle)

Word-of-mouth is a powerful way to make the third step, but also for step 1.
Marketing (of both the traditional and new kind) is still key, as long as it considers all three steps.
Runkeeper + this forum is an excellent example of this. Keep up the good work!