I want to propose something you may find radical: that spending time on Facebook is not only not a waste, but a good influence on your writing. I hear you thinking of the fifty ways it is bad for your writing: the many varieties of distraction, the dumbing-down of social intercourse, invitations to sleaze and prurience, absurd brevity of newsfeed posts (who actually "reads more"), and the five-second-attention-span atmosphere, when your considered comment scrolls into "older post" oblivion.
But consider this: FB offers some of the best writing prompts you will ever read. Especially if you remember to select that "older posts" link and keep reading. Also, if you read selectively and hide friends selectively, the way you unsubscribe when a magazine starts piling up.
I've made poems from comments on FB, read fascinating articles via links posted by my well-read, scholarly friends -- articles that have introduced me to new poets or reintroduced me to favorite ones -- and of course, connected with editors who have published my work, possibly more readily because I am a known (friend) quantity. FB has given me insights into the process of editing and rejections, provided a place to discuss theoretical issues, and even a workshop area in which to write collaborative science fiction satire. (You know who you are!)
Forget about FB's annoyances and distractions. Or enjoy them, But also think about FB as a writer's dream tool. Using it selectively and with discipline can take you in a breathtaking short time on a stimulating journey from wit to snark to edification to meditation to composition to publication. FB reading can be a vital part of your daily writing process. Just be careful, thoughtful and generous in your friending (is that a word now?) and -- as in all human interactions -- give more than you get and you will get what you need.