Out of boredom & curiosity, I had a look at some forum memberhip statistics.
All data is from the members list & the accuracy is only as good as the data in the databse.

Registered members. .............................................................8,460
Members with 0 posts. ...........................................................4,198
Members with 0 posts who joined more than 1 year ago. ............3,610
Members with 0 posts who joined more than 2 years ago. ...........3,049
Members with 0 posts who joined more than 3 years ago. ...........2,418
Members with 0 posts who joined more than 4 years ago. ...........1,772
Members with 0 posts who joined more than 5 years ago. ...........1,074
Members with 0 posts who joined more than 6 years ago. ............. 187
Members with 0 posts who joined more than 7 years ago. ................ 0

There are two memberships with 0 posts that date back to the 19th of February 2002 & these are the oldest. There is no recoreded 'last login' date for either, so presumably they joined up & never looked back in again.

That's potentially 3,610 perfectly good usernames that are tied up & doing nothing, that have never posted in the year or more since joining.
I wonder if it's possible to set a limit, perhaps even two years. If nothing is posted at all in two years, then chances are that nothing ever will & perhaps these memberships could be cancelled & these usernames released to become available to new members, particularly the usernames with a 'Dat' reference.

Just a thought.

EDIT
There are 1,257 memberships that are more than 5 years old where only 1 post was made. The oldest of these pioneers joined up on the 30th of November 1998 & last had a logged-in look in on the 28th of December 2005.
His only post.

The oldest recorded membership is Pedro Branco who signed up on the 27th of November 1998.

His brother Daniel didn't sign up untill the 4th of December 1998 & his is the 5th oldest membership.

The 6th oldest membership in the database is Aussie based Dattodude, from the 6th of December 1998, however the forum existed in another form before this one & Dattodude was involved right back then under a now forgotten username I believe.

Isn't this interesting.