Great Idea!
If you wanted to monetize, but still keep it free (of course, you could and should have paid tiers too), you could send back an additional 255 characters of ad-text, with the agreement being, "you use the free service, you are obligated to display the ad-text somewhere, if it's a web app..."
Advertisers pay for the ad-text.
You collect the revenue.
Wishing your service luck!