Hell if I know. Give what feels right to you - imagine the service going away tomorrow, and someone saying "If only you'd paid $X/mo, this wouldn't be happening!" What's the value of X where you'd regret not having done so?
Not really worried about funding its existence for the sake of it just not freeloading my use while it's here and I decide it's worth it which is what the gp was referring to as well.
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.
