The quality of what you receive depends on the quality of information you provide to any designer or firm. Our suggested process for a bootstrapped startup provides a solution for how to formulate a thought out design idea and get you going in the right direction at no cost.
In reply to your comment about Facebook: Zuckerburg knew minimal about 'design' so he did what so many other startups in the Valley do. Go to the big design firms. This solution works well for startups in the position to hand over the big bucks. Different solutions work depending on startup condition, status, and/or funds available.
This process may work best for startups with limited resources and limited funding. Even if you dont like the initial work you receive from 99 designs or any other design marketplace, two important things can happen:
1. You'll be a better judge for what works and doesn't work for your startup.
2. You're design idea will strengthen from concepts created by these designers at a price you can afford to pay.