Maybe. These policies can be a double-edged sword. It really depends upon how they are implemented. Most of the ranking systems - and it sounds like Facebook's - rely on the immediate manager to do the ranking and the manager has a lot of leverage. It can become arbitrary. It can often come down to how much your manager likes you (outside of your productivity) or managers protecting turf. Enron had a system like this for dishing out bonuses and it boiled down to nonstop political battles to get better ratings.
IANAL but the state that you choose to organize in won't have much bearing on how much income tax you owe. You will owe tax based upon the state(s) that are your legal residence.
Yeah, just talk to a local insurance broker and you'll get set up quick. Make sure you know what kind of liability insurance they are requiring general liability or professional liability (sometimes called errors and omissions). General liability + workers comp will probably cost you a few hundred bucks a year. Professional liability will cost a few thousand.
There are websites that will do it. My state even has a website where you can register with a credit card in about 5 minutes. Doing that first step by yourself is fine. I had my lawyer do it and I found out that all they do is have a paralegal go to the website. However if you have more than one founder you'll want to go to an attorney and work up some sort of agreement among the founders. Right of first refusal to sell their share, what happens to a founder's shares if he prematurely meets his demise, what happens to a founder's shares if he gets divorced, etc. There are a whole bunch of these situations to consider and you need to get the answers in an agreement up front.
I am not a tax lawyer but the issue of foreign taxation is more complicated than 'organize in the caymans and save lots on taxes'. You're still going to have to pay taxes for any business that you do domestically. If you have income domestically you pay income taxes domestically. If you have domestic sales you pay domestic sales taxes. If you have international sales there are all sorts of complicated rules about where those sales must be recognized. Most of the large corps that use this technique are doing it to shield foreign income from US taxation. They can park it offshore and reinvest it internationally without US taxes. Once you bring it back to the states you have to pay income taxes on it (I think you get a credit for foreign tax paid). So if you need your foreign revenue to pay the domestic bills you're going to get hit. Also I believe that a couple of years ago Congress made it more difficult to do this sort of thing after it started impacting tax receipts.
The actual design and implementation of your data model and application are going to be far more important for performance. My suggestion would be to start on MySQL and migrate to another platform if it becomes necessary. You also have to ask what kind of skill sets do you have around and what are you likely to have in the future. A well configured and maintained installation of either platform will out peform a bad one. So are you windows geeks or linux geeks?