Don’t do it.
Facebook private messages
are viewed
by many people
as trusted friend to friend
communications,
not impersonal business to prospect
communications.
Post on your page or profile
instead.
But-but-but, you say,
Facebook limits
the reach of those posts.
Then post multiple times,
in different ways.
I post a book link at least
once a day,
talking about my books
in different, fresh ways.
Don’t betray your prospects’ trust.
Don’t use Facebook PMs to sell.