”What is there to prevent “letmeinfacebook” from being the new most common four word password for Facebook accounts”
Chance. XKCD 936 says to choose the words at random.
Diogomonica is wrong. And so was Bruce Schneier, and for the same reason – he missed that the words are to be chosen at random.
This means for example picking a up a few books and selecting pages and words at random. I picked a poetry book and used only words starting with an "o". Not optimal, but nice.
Password managers are better, definitely. So sure, mention the password manager first. But nine out of ten of your readers will not install them. What will you tell them? Nothing?