What happens when they get all the coins? Do they get to make pos off themselves?
2 biggest wallets are Cryptsy and Bittrex. Just create cryptsy address, send some coins there and ull see your address in one of those wallets in some time.
Exchanges do not stake coins. Its also easy to check: just look at transactions in one of those biggest wallet address, no pos blocks there.
Pools (proly) and private wallets stake.