I disagree.

I think the reason you see those kind of posts is that those kind of applications are low-hanging fruit. The development of Yet Another File-Sharing App or Social Media Service can be accomplished by anyone with a "Learn Rails In 21 Days" book and a credit card.

Companies that are also getting tons of funding right now are biotechnology and solar energy startups, but I have never seen anything along the lines of a "Ask HN: Review my research on getting 3.3% more current per solar flux unit by doping gallium with boron" post, despite the fact that such an accomplishment would likely garner extremely good investment and/or business prospects.