I think that the uncomfortable future for a lot of the e-properties that people enjoy happens to hinge on the fact that the way they are the most useful clashes with the ways they can make money. Big time.
Facebook will sponsor stories, alienating users a little for eyeball time.
Twitter will make sure promoted things get their eyeball time by crushing 3rd party (alienating users).
Digg will- well. You know.
All of these moves hurt the utility of those companies. All of the companies know that.
But something happened with Digg, didn't it? The models of the above have to very carefully balance utility/pleasure of use with a certain amount of intrusiveness to make money because they have found few other good ways, though some get credit for experimenting.
And every time a major e-property does this they make a chink in their dam, inviting their sweet sweet reservoir of users to look beyond into a river of startups. Maybe there are more useful options out there for little fish. Cleaner spaces, open air. Let's be honest I'm bad at metaphors. Moving on.
Myspace became ridiculous and thousands (millions) fled to Facebook.
Digg became ridiculous and many fled to Reddit.
Twitter and Facebook are trying to monetize more and will become more ridiculous and less useful in the process. They might survive, but they are inviting others to take their place in the meantime.
And someday those new companies will build dams around users and either try to get them to gawk at more ads and maybe fall in the same way. This model seems to work for a little while, but they still need to solve the large clash of interest brought about by advertising. Search engines still do it best: ads displayed that try to match queries with relevant content aren't necessarily offensive to taste.
Almost everything else is still trading user abuse for irrelevant product. All users wanna do is talk about the kind of sandwich they ate. Nobody wants to "Learn how YOU can help Hyundai make August National Fuel-Efficiency month!" (real ad).