Zapala is a town in Neuquen province that can probably be classified as the south of Argentina, which implies that hitchhiking there sucks.
At 23:00 there is a collectivo to Neuquen, and it stops if you wave clearly enough. However, Neuquen is not a much better place for hitchhiking.
RN40 South toward Junin de los Andes/San Martin de los Andes
There's a round-about in front of the Casino Black Gold where pretty much only local traffic passes. From there, you can walk along the road out of town for about 800m just past the turn to the railroad tracks where some cars slow down a little bit. Cars pass quickly, but there's plenty of room for them to stop, very little traffic, and they'll see you from far away. A sign showing desperation (i.e. "donde sea") might help.
RN40 North toward Chos Malal/Malargue
If you're looking for a real adventure, you might try to hitch further north on Ruta 40. Coming from down south, be aware that this 550km stretch of the Ruta 40 is the least trafficked and most remote (Tres Lagos<->Gobernador Gregores might seem crowded against it). Even in peak season, do not expect more than 20-50 cars a day taking the full route. Most of them will be packed already and not able to take more passengers. If somebody stops taking you further north, consider seriously taking a hitch, which is not even taking you to Chos Malal, as it might be hard to get back in case of desperation trying to get further north. However, if you're lucky, you will be rewarded with one of the most remote scenic highways of the world. Between Zapala and Chos Malal the road is reasonable, between Chos Malal and Malargue it gets dirt or almost non-existent. Expect serious delays after severe rainfall. Assuming most drivers would be tourists travelling the entire RN40, it might be better to travel north in late summer/autumn (when people come from the already cold south) and travel south in spring/early summer (when people are eager to arrive to South Patagonia or having an adventure on their way to Bariloche).