There is no way to charge for content - you have to rely on the user both having flattr installed and being willing to give you something.
As a content producer you have a direct monetary incentive to not link to or recommand others who also use flattr, as it lowers your cut. The service thus have to work against network effects - the more people who use the service, the worse of I am. At the same time there is no point in installing it on a website, so it has to fight network effects just to get accepted.
I doubt this will be around for long.