If someone writes a line of code against your API today, it should work seamlessly and similarly in a decade.
Salesforce, whose force.com platform my cofounders helped make, sees things this way, and we’ve internalized that approach at Keen IO. API v1 was released a half decade ago. We’ve swapped out nearly every piece of the underlying system since then, but the API hasn’t changed. Calls to API v1 still work as advertised.
Twitter and Facebook on the other hand do not, and have created a legacy of abandoned API endpoints and end-of-life events breaking real business code. The developer community has a long memory for these sorts of infractions. If what you sell is ads like at those companies, maybe you can get away with this. If what you sell is the API itself, you should version strongly and deprecate rarely (or never!).