With the Ethereum Metropolis hard fork coming up, what happens to ICO tokens like Steemit and all the others built on top of the Ethereum network? Do they automatically fork too?
The tokens automatically fork, too. A fork means there are two separate copies of all past transactions (the ledger). New transactions will be added to one (or both, if the fork does not take advantage of EIP155 and the transaction is otherwise valid according to the rules of both forks) copy of the ledger. If the fork is successful in the sense that every miner upgrades or no miner upgrades, then only one copy of the ledger will receive updates. Then, separately, anyone who wishes to transact on Ethereum, can choose which fork they wish to use and ascribe meaning (and value) to.
In the fork after the DAO attack, there were miners on both chains, leading to Ethereum and Ethereum Classic. It was expected by the core developers that all users of Ethereum pre-fork would upgrade and thus remain on Ethereum, even if there were some "stubborn" miners on the ledger that adhered to the old rules. In that sense, I think many/most people thought that ETC would be extinguished because no one would ascribe value to new transactions on the ETC ledger. In theory, the DAO tokens could still be worth something on the old ledger (maybe as a sentimental token?). In practice, if a company were tracking its shares on a blockchain using a token, the company would probably pick one chain or the other as a single source of truth (although one could in fact keep using both chains and maintain value on both). Similarly, yes, the tokens may be valid on both chains, but support will likely be dropped on one of the chains.