On the very first page of http://wordpress.org/extend/plugins/leenkme/, the last paragraph reads, “You will need a leenk.me API key to use the leenk.me plugin. The leenk.me subscription is 29USD per year with a one month free trial. Cancel anytime before your trial ends and you won’t be charged. This small subscription fee is used to maintain the leenk.me server and for continued development/support.”
The leenk.me plugin is completely free and open source. I encourage anyone who is interested to developing their own backend to do so and use the leenk.me plugin freely, if they desire.