I highly doubt that anyone ever writes, "I think Joe is the Mole" in their diaries. We've seen a few - most of them are Who sat Where, what order tasks went in, who won/lost challenges, what people were wearing, who ate what at dinner - things to prepare for a quiz. The MOST they are going to write is, "Clay seems suspicious". It doesn't make a whole lot of sense to simply put who you think the mole is in your diary - you KNOW who you think the mole is, and we've seen in the past that sometimes people get to see your diary.
And even so - I can't imagine that the producers would contrive time for the Mole to see diaries, when they could easily look at them themselves if they wanted to, and feed whatever relevant info to the Mole if necessary.