Belgien, thumbs up!

I agree completely. Belgium is a great country to hitchhike on motorways, national roads and sideroads. Recently I took a short break on a service area, walked back to the filling station, saw two Muslims doing prayer direction Mekka at the backside of the restaurant, a van appeared, put up my thumb and off... Couple of days before we had on national / regional road hitching a civil servant in duty drove us tens of kilometers further than his destination "because I am still in my working area" over a very scenic route... Last summer ~ I'm not in Belgium every week ~ a driver with the whole family made (2x) 25 kilometer more to get me back to Maastricht...

Hmm, meine Erfahrungen in Belgien waren genau das Gegenteil, allerdings war ich nur im französischsprachigen Teil.
Wir wollten zu zweit von Mönchengladbach nach Paris. Bis Lüttich ging das auch ganz gut, ab da war es fast unmöglich. Letztendlich hat es 2 Tage gedauert bis Paris.

Ich vermute, dass das ganze im niederländischsprachigen Teil leichter ist (wie in den Niederlanden) und im französischen Teil entsprechend schwieriger, wie in Frankreich.

Kann das jemand bestätigen oder wiederlegen?

What I can is make a short analysis of my own experiences in Belgium. Excluding rides of over 100 km...

In Flandres had 34 rides, average wait 8.35 minutes, average distance 31,1 km, wait per km is 26.8 seconds.
In Wallonia had 54 rides, average wait 12.5 minutes, average distance 28,7 km, wait per km is 43,6 seconds.

And including >100km rides...
In Flandres had 35 rides, average wait 9.0 minutes, average distance 33.2 km, wait per km is 27.1 seconds.
In Wallonia had 58 rides, average wait 15.3 minutes, average distance 44.9 km, wait per km is 34.0 seconds.

Indeed, these statistics confirm your statement that waiting times in Wallonia are longer.

If I exclude rides >100 km and rides in Liege ~ a city which I consider "difficult" to hh and which is on the way of most southbound journeys from my hometown the mark for Wallonia would become:

In Wallonia had 43 rides, average wait 11.7 minutes, average distance 30.9 km, wait per km is 37.9 seconds.

Still Flandres ranks better then.

Wow, that is still much better than my experiences.

We were waiting at the service area in Liege for 2,5 hours, then we got a ride to Brussels (we took it although we wanted to go to Paris, because it was already dark and very cold and we didn't have much hope).
The next day we got a ride from Brussels to somewhere in the south of Belgium after 15 Minutes waiting. Then another 2 hours of waiting until a trucker from Germany stopped.
Maybe we just had bad luck, but we will cross Belgium again in July, maybe it works better then.

On Belgian motorways I'm usually asking for rides directly. As I speak both Dutch and French that works well for me. As a whole my statistics tell that my avg wait when asking is about the same as when using a cardboard sign or only thumb. In reality though, asking scores much, much, much better, since I use that technique always in more difficult situations (little traffic, uncommon direction, darkness, rain).

Of my 21 rides longer than 200 km I got 13 on petrol stations. Of those 11 by mainly asking around and 2 by using a cardboard sign. But situations with cardboard sign were in Luxembourg, where almost all Dutch leaving would pass by my Maastricht on the way (and there are many, because of the lower petrol prices in Luxembourg). Further 7 times more with cardboard on places where asking would be inefficient, impossible or hard: three times on a national road, twice on an official hh spot, twice just after a non-Schengen border). On these spots way more drivers pass by than at the end of a motorway service area.

Once I got a >200 km ride on a petrol station after I took my cardboard sign from my bag and fixing it on the backside of my bag, so people could see my destination. In a minute a driver told me that he had seen me putting on the Berlin sign and offered me to go along to Berlin.

Just remember it took me a year to understand many basic techniques of hitchhiking and to overcome the "fears" of asking for rides and all the time I develop the techniques to (literally) open doors.

We tried to ask people for rides, but that seemed to be absolutely senseless since both of us don't speak french and noone in belgium spoke english and they refused to try to understand us (in other countries I could always communicate somehow with gestures etc).

Later we found out, that many people can speak English, but they just don't like it.