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My sense is that Facebook is really well made, and isn't going anywhere (i.e., Facebook is to social networking what Google is to search). I could see LinkedIn getting destroyed, though. They fill a very real need, but don't seem to do it very well. This is all very qualitative, but I never feel like I have a pleasant experience on LinkedIn.
Facebook is absolutely fabulously done (complaints about it concern privacy and abuse potential but the interface is top-notch). The thing about Facebook though, is A) The interface requires lots of care and feeding to stay clean and appealing - Facebook has stayed nice through it small number of very talented engineers. B) Facebook's economics and its supply of topnotch talent has been leverage from its constant gro…
The iPhone app is worse. You can't even see your own lists at this point, and clicking on youtube links doesn't just play the video, it opens an embedded browser that gleefully suggests that you try adding youtube to your Home screen.
The iPad optimized site is so useless for reading anything a friend links to that I don't even bother clicking anymore, I just wait until I am at a PC.
The message viewer makes it impossible to quickly mark as-read or delete the vast numbers of messages that I never open because the popup notification on my phone contained enough information to never need to open the full message.
The reason that facebook is still going strong is not good user interface or experience, it is that it is incredibly sticky. They managed to become the new email for a lot of people, and that utility is enough to force a lot of people to grind their teeth and ignore the horrible parts.