The first one is Upbit (korean exchange, has always been in the top addresses), the next 3 are vesting contracts for the DAO and the Foundation, then Coinbase and Binance use several addresses to hold their tokens but are the exchanges that hold the most MANA tokens. Following are Huobi, Bittrex, Gemini, Bitfinex and Poloniex. Most of their addresses are not labelled on Etherscan.
If you want to play blockchain detective I recommend you to use better tools than Etherscan, like https://nansen.ai