[roughly]

I'd still argue to give the amount the service is worth to you, but if you're not willing to do that, then I'd say bilal4hmed is probably right - $1/mo is probably sufficient. Facebook, Twitter, etc. all have ARPUs at or below $12/yr.

[bilal4hmed]

also check if your employer does a matching donation. Its an easy way to double your donation. I do a single yearly donation for that reason