whats stopping the Charity-Water to make a fake twitter account and do Give 1000000 Twollars @username.
Also, you need to give an incentive for the businesses to use this, instead of sending the $$$ directly. For that you'll need to build up your core group of users, and then tweet stuff like "IBM has donated $15,000USD to _____ Charity thanks to your efforts"
Every Twitter account has a starting balance of 50 Twollars. So if you would tweet that - you would get a message saying you don't have sufficient balance.
The incentive for the businesses is exactly what you're mentioning - the visibility on Twitter.
Also, you need to give an incentive for the businesses to use this, instead of sending the $$$ directly. For that you'll need to build up your core group of users, and then tweet stuff like "IBM has donated $15,000USD to _____ Charity thanks to your efforts"