Social fork here means running some other software basically. Imagine Vitalik saying "Ok, since coinbase is being evil, let's all move to a new Ethereum-without-cb that is a copy of Ethereum but their ETH is removed. To do so download the following software that follows a new Ethereum-without-cb chain instead of the Ethereum chain. And if enough people agree, we'll probably just call that just Ethereum (like the original Ethereum is now called Ethereum classic, and the current Ethereum is actually forked).