If I were they, I'd steer clear of directly targeting ads towards private matters discussed in the diary. Creep your customers out too much and they won't come back.
How do you handle intent though? When I see that email my goal is to write a new entry, not click on an advert for Deepest Darkest Secrets (TM).
To fix the intent problem, perhaps showing relevant ads while you're writing could make sense. If I'm writing about health and financial matters, that stuff's in my head and I might be coerced into clicking on a related ad.
Email CPM ranges from $10/CPM to $100/CPM depending on targeting. http://archive.newsmax.com/mediakit/newsmaxemail_rates.cfm
I'd imagine people would put health and financial matters in their private journal.