You could just copy someone else's agreement (not sure if they are covered by copyright though). I'd just summarize the x main points and put them large and bold above the signup button:
- You agree to give us feedback and report bugs
- Assume that all your data will be public though we do our best to keep it private
- Assume we will lose all your data at some point
- We might go offline at any time and never come back
- Treat every data we give you as if it were wrong, because it probably is
The first item is important. People like to be consistent and it should increase your return drastically.
Note that I'm not a lawyer, the right answer is: talk to a lawyer, and these terms are probably not bullet-proof.