>I noticed, in Argentina, that this for is not only used for friends, but also, a lot, among people who see each other for the first time.

Yep. This is increasingly more common among young people. The main change happened around my own generation, I think.

Indeed, I hesitate sometimes when I'm introduced with some people slightly older than me. Calling them "vos" can sound too informal, but sometimes calling them "usted" can sound as if one is treating them as ancient. 8-)

What a mess, isn't it?