I have several friends who have accounts on multiple web applications (Facebook, Flickr etc). However, on my lifestream, my friends' updates get split up, so all their different updates are split up.
As people may use different email addresses for different services, I don't think that you could auto-detect for it - but perhaps you could have a set of options underneath the user photo so users could manually link them?
That way, if you didn't hide the usernames, people could use it as a sorting mechanism (eg, friends, family, colleagues, etc)
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