I've thought FB would be good for promoting my upcoming book, but based on my casual observation of my 124 "friends", I wonder.
They include a few people I "knew" (some by name only) in elementary, junior high, and/or high school, with the majority coming later in life, mostly business acquaintances.
From the look of it, maybe 10% account for 90% of all posts, most of that having to do with what they ate for dinner, their politics, or whichever team just won/loss whatever sporting event is of interest to them. In short, a pretty boring group, which includes me since I am part of the 90% who say very little.
To me G+ seems more grown up if not less personal. I don't encounter many people I knew in some other life; most everyone I follow I "met" here on G+. I wouldn't mind connecting with more real friends but few I know are here. They're all on FB LOLing each other.
Many FB'ers say they know about G+ but don't come here 1) because everyone they care about is already on FB, and 2) because what they see here is too serious. (Hearing that makes me want to LOL, which I can't do, hating that phrase as I do.)
So, promoted posts or ads? I'll have to learn more about who each would reach before deciding.