instagram definitely uses the `website` field if it's there - example in https://github.com/snarfed/activitystreams-unofficial/blob/master/instagram_test.py - but it does fall back to the profile url otherwise. i *think* all of them should do that; definitely let me know if not.

re non-bridgy users, i think it might happen when a bridgy user retweets (reshares etc) a non-bridgy-user's post. maybe other cases. #51 is a possible future (but not current) case.

regardless, no need to do it. pls just include a comment mentioning the possibility.