Don't take this as a disagreement with your whole comment (it isn't), but one trap that people fall into is equating what people say they want with what they actually want - at least as expressed by what they are willing to pay for, which is what shareholders and VCs care about.
There was an article on HN a day or two ago about how asking people to bet on political propositions affects their stated beliefs - when there's no cost, people will happily spout whatever their favoured party's line is, but if you ask them to put their money where their mouth is, they will often skew away.
VCs are investing based on where they think people will actually put their money, not on where they say they will. If evidence is that people spend a lot of time social networks, AND evidence is that advertisers will pay money for web adverts to people, then it might follow that a new social network is a good investment.
The main issue as I see it is that as we use money as a proxy for value, we disproportionately weight the desires of the rich over those of the poor. A service for rich people (e.g. an online photo sharing website) doesn't need to produce much utility to be worth a lot of money, whereas a service for poor people (e.g. clean water for poor villages) can produce a lot of utility and still be worth little money.