I think we see more value in having projects related to the projects. The first example you give: "you rock" is more meaningful in the context of a project. You can leave that message in a project or love the project. The second example you give "want to be my friend" is implicit when you add someone to your list of friends. The other person is notified that you have added him or her and it's up to them to decide if they want to add you as their friend. Don't you agree?
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I think that the comments on projects mechanism works fairly well, though there may be use for comments to users about things other than projects---like ideas for new projects, requests for collaboration on projects, and general chit-chat.
The current "friend" metaphor is so poor that adding someone to your list of friends is totally meaningless. All it provides is a slightly quicker link to their "my stuff" page. It is past time to rethink that metaphor.
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