Okay, technically it didn't go down. You could still read everything, but not update.
When I tried to comment on some posts this morning all I got was a spinning wheel
I tried it on a couple of computers and my phone with no joy. Eventually I checked in the Steem Chat and someone posted this info:
Two bugs have been identified by dan (and changes to fix them committed to the develop branch). One of these bugs allowed for a bug in a plugin to incorrectly propagate up and affect the main consensus code. And that's what caused the fork: a bug in the follow plugin became much worse and caused a fork because of that second bug in the main code. Most top 19 witnesses were protected from this forking event because they run nodes without any plugins enabled. A temporary workaround for those running v0.13.0 and are on the incorrect minority fork (or stuck on block 4,383,207 after a --replay) is to run their steemd without the follow plugin enabled (don't forget to --replay).
On other social sites if something goes wrong they can just roll back database changes to fix it. Steemit uses a blockchain and anything that goes in there is permanent. So they have to deal with any potential problems. The site is back now, so I hope it's all okay now. No software will ever be perfect. It's how you react to issues that matters.