With vices I suspect that it isn't just the activity concerned but the purpose, which varies from person to person.
e.g. TV can be used to educate but it can also be used to distract
That said, some vices may be worse than others. In the case of social networking, I guess the obvious alternative is going to parties which are OFC a gateway to alcohol consumption.
>some problems in mathematics will remain beyond the limits of quantum computing
I don't think it's surprising that quantum computers, by themselves, will not be able to solve mathematical problems. Problem solving is not a feature of hardware per se, it is done by humans. Perhaps one day by AIs, too, if we can discover how to program them.
Two tantalising potential benefits of Quantum Computers:
(1) the ability to model all kinds of matter efficiently at the molecular level which will spur the development of nanotech
(2) people will ask 'Where are these humongous computations being performed?' and then embrace the 'many worlds interpretation' of quantum mechanics, which will in turn create further scientific and philosophical progress
Well, they're speaking in rigourous terms. Quantum computing will allow us to attack an enlarged set of problems in polynomial time. For example, integer factorization. However, there are still going to be other problems that quantum computers cannot solve in polynomial time (ie, much better than classical computers). For example, it's generally believed that they will not be able to solve NP-Complete (traveling salesman) problems in polynomial time.
I agree with your optimistic stance however everyone has at least a small amount of valuable knowledge. I trust we will extend the lives of all those wish to remain alive.
Since you're an optimist, you'll probably be one of them :-)
dasil003 doesn't say what the ethical question is, but I think he's got it the wrong way around with motivation. As Aubrey de Grey has argued, it is the fear of death, coupled with the pessimistic assumption of death's inevitability, which makes talk of life extension taboo in many circles.
('If you can't avoid something unpleasant then embrace it' is a viable psychological strategy.)
One ethical objection to not researching life extension is that huge amounts of healthcare budget go into extending the lives of frail, sick people in their final years. When we know how to keep people healthy indefinitely this problem will be solved, although pessimistic individuals will then have to take the responsibility of choosing when to die (if ever).
There are plenty of activities that may seem to an outsider to be a waste of time. Daydreaming, for example.
(Just don't tell that to Albert Einstein who used to daydream about travelling on a wave of light.)
Even supposing that social networking is some sort of vice, it's easy to think of much more harmful alternatives.