The difference with sites like Digg etc. is that they are specifically platforms for sharing and the implication is that you will sharing other peoples stuff. You would not be called a spammer if you submitted hundreds of links to other blogs etc. You are only considered a spammer if you fill them up with your own.

The point of aggregation, however, is that you are gathering all of YOUR stuff in to one place so that it can all be accessed together rather than jumping off to multiple sites. Even if you don't like or comment or even follow anyone else how can using a core function of FriendFeed be considered spamming?