Several years ago - before Free Republic was split up into several fora - traffic statistics used to show a 75% drop on weekends. Maybe 90%.
On the news/activism forum, traffic still drops severely, although I can't prove it with numbers. All I know is that very few articles are posted there on weekends, and even good ones get less than a handful of comments. The sidebars are mostly the same on Sunday evening as they were at quitting time on Friday.
We have about 30% fewer posters on the weekends than during the week. There are about 35% fewer articles posted, but only about 25% fewer replies posted.
Traffic drops off 15-20% on the weekends.