Which is worth pointing out - it is not a social payment service, it is a social donation service, since you get to view the content whether you pay or not.
There wouldn't be much difference, but taking donations is only a small part of what paypal does. You can donate money with a credit card, but that is not what it is mostely used for.
Flattr on the other hand, can only be used for donations, which means that it has little, if any value.
Also, it's hoping that people will donate - if nobody cares to donate any money, would this mean the content creator gets zilch?