Well, cough-cough, I haven't seen my 20s in a while
but I do have a few (and by few, I mean 3) close porteña friends. I met each through activities. The first I met through riding horses, the second I met in NY before I moved down here full-time and the third was my Spanish professor.
I have a lot more accquaintances but that's a world apart.
Making friends isn't easy anywhere after college but I do think it's especially challenging here. In most big cities, everyone is transient (ie, not many people actually grew up in NY or Boston or LA - lots moved there after college). So everyone is in a bit of the same boat. Whereas in Cap Fed, I feel like no one ever really leaves so you have social circles formed early that really don't change.
My only recommendation - if you work in an office, it makes it a little bit easier (ie, all my employees go out together after work for after-office). If that's not an option, activities are a good way to meet people. And since you already have a shared interest, it's a good ice-breaker. Yoga, tango, drawing, riding horses, whatever it is.