There's one rather large difference between Hacker News driving traffic to Techcrunch, and FriendFeed driving traffic to twitter. Hacker News doesn't actually scrape techcrunch or get it's content via an api.
From a practical viewpoint (and this is why I used the word parasite - not to be pejorative, but to indicate that FF is entirely dependent on its hosts), Friendfeed could be shutdown tomorrow by the sources. If they're using APIs, then just about every api out there is for non-commercial use, and if they're scraping, then an appropriate robots.txt, backed up by IP blocking and lawyers, does the trick.
P.S. Since you didn't seem to notice, "perhaps" was sarcasm on my part. FriendFeed will most certainly create some additional traffic and/or visibility for the sites it "leeches" off of. It is important, however, to note that traffic != visibility. Sometimes times visibility detracts from traffic because there's no need to visit the source (i.e. Google Maps, YouTube, Scribd, etc.) but other times it creates traffic because the source provides added value (i.e. Thumbnails -> Larger Image, HN Links -> Stories, etc.). It's also worthwhile to note that the former (increase in visibility, decrease in traffic) still has benefits, as your service becomes more well known (creating future traffic). I'll leave you with this: http://friendfeed.com/e/d1ffcc73-040a-5609-6168-993c4549591a