Whilst you might not create a stable business out of a Twitter feature, if you can implement a feature that lots of people want for a time when no one else provides it you could make a lot of dosh in the short term.
3rd party platform + a feature that everyone wants + makes money... it would make zero sense for the 3rd party to sit on their hands. you're either going to be acquired (lottery) or, more likely, they will implement the feature on their own and you'll be both out of a revenue stream and have a piece of obsolete software on your hands.
good for hackers with ADD and I see nothing wrong with building this kind of thing for teh lulz etc, or to promote your real app.
very strange when you see VC-funded "features" though.
The StackOverflow folks have the right idea - I think the market can support a bunch of niche Q&A sites with tight communities.
For monolithic Q&A sites on every topic under the sun, you need massive traction otherwise you have the usual chicken/egg UGC problems that we've seen a million times before. Facebook already has massive traction, Quora doesn't. Game over.