For some people this may be the first time they ponder relying on free third party services. Are they reliable enough, can we count on them, can we trust them?
The fact of the matter is that when you ask anyone how much downtime costs, the answer is always higher when you are in the middle of an outage. Ask them when the service has been running just fine for a few years, and the financial figure gets much smaller.
So with Twitter under a denial of service attack right now, lets take some of that "free" time to implement some common sense guidelines to protect your instruction and courses.
1. Diversify - Just like your 401k, you need to have a variety of services you can use. If Twitter goes down, move the in class responses over to a wiki or to a Wiffiti (which can still take sms message from cell phones). Technology will never be 100%. We can get close, but there will always be that .001% of the time that it is down even on the best of systems.
2. Always keep a backup copy - It sure is easy to save something in Google Apps or on Gmail, but having a backup copy that you periodically save to your network storage at work is always a good idea. If your whole course for a given day revolves around a few files, they probably shouldn't be stored in one place. You can either be stranded from them by an Internet connection being down, or the provider's service can go down.
3. Adapt - One of the fine arts of being an instructor is being able to teach on the fly when you must. I don't know anyone who has walked into every single instance of every course they have taught being 100% prepared and mapped out. If you know the material (and you should...) then be creative and kick it old school! Pick up a whiteboard marker, divide the students up into discussion groups, go sit under a tree outside, whatever it takes. Teach your students how to react when technology is unavailable, its a life skill they are missing out on! (Don't believe me? Just ask a group of 17 year old students what they do if their car breaks down on a rural road. Then tell them their cell phone is broken...interesting responses.)