As others have commented, Flattr itself keeps 7.5%. The detailed fees are described here: https://blog.flattr.net/2017/10/our-new-fee-structure/ Hopefully the payment cost will go down again, but unfortunately micropayments come with high transaction cost. Compared to revenue streams like App-Stores or even advertising, this is still much more efficient.

Regarding keeping money in the system: In the new version, there are separate accounts for "givers" (contributors) and "receivers" (creators / publishers). So, just shuffling money around is neither the obvious nor the intended solution.