Contact data is relatively uniform - you got names, addresses, phone numbers, maybe a few dates. Even with the handling of different addresses per contact, different systems already show divergent behaviour, even when they use the vcard standard.

This becomes immeasurably more difficult with information from, let's say, Amazon: A competitor would not have the articles I've viewed, or the comments I have left under questions, or a compatible rating system.

Even social networking sites ... how useful is it to import twitter exports of my tweets answering to someone if neither that someone nor the content I answered to is available in the target system?

Sometimes, it is by design - in the majority of cases, it's just different people implementing different use-cases differently.