For a blog or regular website, I used to just push the updates - no need to warn anyone!  But for a SaaS, a user can be in the middle of a task that writes data..so it's more annoying to pull the plug. I think my downtime is about 10 mins for the update to go through.    What's the best way to let people know?    Send an email?  Update Instatus?  Send a tweet? ...