It really depends. Ideally, these guys you are asking for advice would either be potential customers or have some sort of interest in your application. Don't email the star street sports guys about a financial startup (unless it's a B2B product they may be interested in).

You'll want to cater specific feedback to each person you email Don't ask Alexis Ohanian technical questions - ask him about branding or design. Also - what do you honestly want feedback on? Don't think of this as a stepping stone towards VCs/Angels, you can get honest feedback about your application that may help you tremendously.

And don't forget to reciprocate. Introduce them to people you know that may be helpful in their business.

Email me if you have any follow up questions rcavezza[at]gmail [dot]com