80% of STEEM that was premined was converted to vests. Vests have to be powered down and paid out in equal payments over 104 weeks. The only way to dump the 80% on the market is if they powered down over two years, and then dumped it. If this were to happen we would have two years notice before it happened.
Steemit is growing, in the future it will have expenses, bounties, etc to pay out. As such it isn't uncommon to have a general fund with premined coins. What is unique is that for Steemit, these coins are not immediately liquid.
The 6 founders split 53,000 STEEM which was awarded as vests. Once again this isn't immediately liquid.