I was really surprised at the number of backpackers doing drugs in South America (other than weed). I think the morals are a bit dubious when it comes to visiting another country purely for cheap coke, which a lot of people seemed to be doing in Colombia, and thus encouraging the trade over there. Colombia, especially, is a country that is trying to move away from that side of its past and many of the hostel owners we spoke to said they feared drug tourism actually setting the country back. Somewhere like Taganga is a prime example of the bad side - a quiet fishing village once that is now a drug-fueled backpacker haunt with violent crime on the rise. Many backapckers don't even see the problem as they breeze in, do drugs, and then move on to the next place.
South America is, of course, a great place to party! We had some fantastic nights out in Rio - the Lapa street party is an absolute must and the caipirinhas are big and cheap! We didn't find it too dangerous but did go in a good sized group. We chatted and mingled with the locals. One of the best nights out we had.
Colombia is also great fun, especially in Cartagena, Medellin, and Bogota.
Not to forget Buenos Aires, you'll have an awesome time, but go to some of the sights as well!!
If you want to party in La Paz, I'd recommend the Wild Rover. We had a great time there, on our first visit, when we met some terrific people ad they have a good bar. The second time we were there was pretty awful but that's a long story!
Oh, and Loki del Mar in Mancora, Peru is another good place to party. You can surf all day and drink all night!
if you are doing coke, be careful out there as we met lots of people who had had frightening run-ins with the police, sometimes violent, after they had been set-up by the drug dealers/police, i.e. you buy the drugs, the police turn up, take you somewhere, threaten you with prison etc, and then you haggle over a bribe. One couple we met were marched to an ATM by the police and told to withdraw as much as they could.