It really depends on whether you look at the startup as the means or the end. For example, while the creation of Microsoft did not directly fix the problem of healthcare or hunger, it allowed for Bill Gates to create his foundation, and thus indirectly created a ton of good for the world.

Same goes for Facebook, yes of course keeping up with your friends has no direct impact on real issues, but the fact that 110m went to public schools in an area that really needs it - that is a pretty big impact.

The same goes for any other person who creates wealth from web startups or any business in general - I don't think that web startups themselves are going to do much directly but when you take the knowledge and potential wealth that you acquire in the process and put it toward a good cause, that is one way that you can fix a social or human problem with your app as the author challenges.

Just my take on it.