If you're an early adopter, you can pretty much count on a service you try out to develop quite a bit between the first time you try it and two years later. But it is still quite rare for the core use case of a service to change. One exception, for me personally, is Foursquare.
When I joined Foursquare, I used it as a check-in and social coordination service. I.e. I'm here, come and join me for coffee.
Today, with three years of usage data, Foursquare has been able to develop the Explore view, a nice looking and relevant view of what's interesting in a city.
As a result, Foursquare has morphed into more of a consumption-oriented city guide or yellow pages service than a creation-oriented social network. At least to me.