Plain and simple, state why you are no longer regularly playing FAF. Please keep it to a simple few statements such as "I stopped playing because I just grew out of the game," "The progress is taking too long and I don't feel like waiting," etc, etc.
For me personally, I am about to stop playing regularly because there is really no competition anymore. The weekly tournaments stopped, the clan war saw about half the clans drop, and top players either are sick of the game or are fed up with the way things are going.
Please keep this as constructive as possible and I ask admins/mods to delete threads that get anywhere over a few statements.