Personally, while I have an Argentine wife and friends, I found most of my expat friends through through my religion. I am one of Jehovah's Witnesses and I attend a local congregation here in Palermo that meets in English. As such we have a mix of people from Australia, Ghana, USA, South Africa, etc. You're welcome to attend any time. Meetings are free. Friday at 730 pm and Sunday at 1:00 pm Armenia 1250/56.
My name is Dominick. I am from Chicago but I have lived here for about 10 years now. I first came here when I was 20. The coffee chat is a good place to start, I just went to my first one last week. But its mostly older folks. I understand what you are saying about missing English speakers your own age...but also not just hitting up party time at the hostels