The best way to give a big THANK YOU to your donors is to comment on their donation when it appears on your Supporter page. You need to be logged in to your page in order to do this!
Simply type what you would like to say underneath their donation, and click "Thank ____."
If the donor has opted to have their details available to you, they will also be included in the automatic email you get when you receive a new donation to your page. This way you can write a longer thank you response by email if you wish to.