Its not that black and white. Just cause your startup isnt directly fixing the world problems it doesnt mean that one day it might not. Either directly or indirectly.
I hate to say it but i think in more cases then not the kind of startups that most of us imagine doing (ie web startups) have a hard time doing anything to solve the worlds problems directly (the worlds problems that you mentioned more specifically). Not saying there isnt (i seen a few that raise money and help schools execute projects for instance but there are only a handful) but for the most part they do not tackle for example educational problems DIRECTLY such as say OLPC does.
I really love sal khans khanacademy stuff, its truly awesome but it isnt getting to the kids that dont have a roof over their school let alone an internet connection and many of these schools could really use his material.
This material might one day with the help of a very rich successful individual who is willing to purchase computers for the less fortunate parts of the world to then be able to view khans stuff. I think this applies to ALL web startups that isnt directly raising money for the problem parts of the world.
BUT what can be done is, the success (monetary or influential) we get from allowing people to find restaurants that are near them, or allows people to search for data easier can be put to good use.
Maybe even purchase those computers for some unfortunate part of the world that will then allow them to view sal's, standfords or any similar educational content that will actually change the lives of those kids.