How to manage communication during application downtime?

Here is what I do:

• State very clearly what the consequences are (right now and in the immediate future). Highlight likely permanent consequences or lack thereof (data loss, loss of employee-hours).
• Keep the tone very neutral. Do not spend energy on blame/guilt. Ideally this conveys "I want to give you information but my attention is also needed elsewhere".
• Your notification will be forwarded to a lot of people, make sure your CEO understands the gist within the first half paragraph. Usually I provide an 'executive summary'. Technical details can provide background information to other technical people.
• Provide contact details (preferably someone who has the time in the heat of the downtime) for further questions, and ask patience in the same sentence (this works often).
• Promise updates when the situation changes.

Send updates when there is good news, before office closing time ("all staff will continue through the night" - account for timezones if necessary) and again around office opening time.

When the issue is resolved (for any definition of that word), send:

• A summary including timing of the consequences
• The actions/changes taken on short term and planned for the future ("lessons learned"); based on:
• Technical root cause analysis

Keep any calls for blame, guilt or lynching in separate mails, preferably after some cooldown time.

Do not commit to anything during the downtime unless you are really, really sure you can deliver. Somehow two separate "bad news" situations are worse than a long one.

I prefer to use a medium where a notification is pushed on every message (mail, Twitter, ..)