I've made two "I'm taking a hiatus" announcements. I tried to do it in the most chipper way possible, but I felt an announcement was necessary because I stayed in touch with a lot of people solely through FB and I didn't want to let the friendships languish.
The announcement was, "Hi! I'm taking a break from Facebook. Send me a PM if you need my personal details. Cheers!"
My ex once did a weeklong thread of "Goodbye, cruel FB" in which he got into a heated argument with an aunt and they ended up severing their ties for good. After a week of him regailling me with the next installment of the drama, I finally said, "How can anyone miss you, if you won't go away?"
He then made a big magnimonious announcement about how so many people begged him to stay, so he was going to grace us all with his continued presence, but outlined his rules that everyone had to follow or he would de-friend them. My friends started calling him the Boss of Facebook and I found the whole process mortifying.