There are the books where marketing material can be posted to one or more Facebook groups. Despite the fact that the percentage of active participants in these groups seems to be well under 5% the sheer size and number of the groups means that book sales of between 5 and 30 copies a month is a perfectly reasonable goal. My astronomy and our grave-hunting books fit into this group.
Then there are the extreme niche books where there isn't a Facebook group when enthusiasts gather to share their hobby. The Biggles Companion (now renamed "The World of Biggles") is my best example of this genre. The only place where the book is marketed is the Amazon site but this seems to have been enough to generate 2 - 5 sales per month for the last few years.
The ghost writer books have been hard for me to promote as ghost writers, almost by definition, don't shout out about the books they have co-authored. The notional authors have been promoting the books to friends, family and specialists in the field and they have fairly consistently achieved 10 to 20 sales per month.
All three of these groups can be regarded as successes but there are a few books I have written where the sales have been minimal. These failures have been books where the target audience has been mainly UK based and the subject matter has been specialised and where there isn't a relevant Facebook group. This "triple whammy" needs to be remembered when planning future publications!
UPDATE - April 14th 2015
This was a well-constructed argument but one with which I tend to disagree.