This is something I always consider when buying my own software but it brought back to me just how important it is for enterprise tools, and how I ought to mention it to clients more!
When we we were introducing tools at the BBC this was one of our most essential criteria. We knew that the churn rate of social tools was high - even then nine years ago and it is somuch higher now. If we didn't want to be stuck with all of our users data left in a legacy tool we had to get it out of the old one easily and then into the new one with as little pain as possible.
Worth keeping this in mind.