Those are the FFC “Pays de Sommières” trails I think.
I don’t think I’ve followed any, but I’ve ridden in the same area – the area between Sommiéres and Nimes in the Vaunage, and there are some very active clubs in Calvisson and Caveirac. Mostly XC stuff with a mix of fireroads (easy to find) and a LOT of natural singletrack (sometimes not easy to find).
The waymarked FFC trails are on this site as GPS traces: http://www.tracegps.com/fr/auteur/50279-1/liste.htm#
There is also the “Pays de Lunel” area just to the south : http://www.ot-paysdelunel.fr/Respirer/L-Espace-VTT-FFC
One of the best things I’ve done this year was the Offroad race in the Vaunage: http://www.offroad-bike.fr/les-balcons-de-la-vaunage/parcours.php
To the west there’s supposed to be good fun in the Beaulieu quarries and around. A bit further there are really good trails around St Croix and Pic St Loup, but nothing marked.
All of that is pretty XC – if you’re looking for more enduro-ish stuff then I’m straying from my area of experience, but I think the Gardiole on the other side of Montpellier should sort you out (Strava is good for showing where the specials go down)
You can also get an idea of where’s good to go from the GPS sites like http://www.utagawavtt.com/ or http://la-trace.com/itineraires/liste/vtt/ but watch out for following traces from “Randos” these are organised rides that sometimes cross private property that’s only open for the event.
Here’s a vid from a rando that started in Sommiéres – skip to about 1m30 to see typical fireroad (and singletrack)
[video]http://www.youtube.com/watch?v=fnMhAZa3fbU[/video]
Actually here’s one from the Offroad event too
[video]http://www.youtube.com/watch?v=FaYCj87NX44[/video]