I added a book to my WL last year after seeing it in my Daily Digest email. That would mean someone posted it. But the last posted date that appears on my WL is 2008. I wasn't a PBS member in 2008 so how is this possible? And I am sure i first saw it in my Daily Digest.

Now I wonder how accurate the other last posted dates are on my WL. I should not that most are simply marked N/A.

i've always accepted them as accurate.

Marya, I have around 35 or 40 on my WL that are like what you described with the dates that I wishlisted it as years before I joined PBS.  Can't be accurate, but the rest of my 500 seem to be.

I wonder if posting them to a friend or simply removing them from your bookshelf also removes them from 'last posted' ...?

I find you can't make assumptions about members based on their status as full members or a la carte. Sometimes a la cartes are very quick responding to requests and full members can be some of the worst.