51xxU! (numeric for 我要谢谢你！= I want to thank you!) is a Google-interfaced site for what is essentially public text-based greeting cards. You have a choice of sending your (registered-with-51xxU) friend a 感谢 Thank You, 祝福 Blessing, 道歉 Apology, 期望 Wish, and 建议 Suggestion.
FYI: 51xxU’s precedent is the popular use of SMS for holiday-greetings, especially during Chinese New Year when almost anyone would be bombarded with templated greetings.