Stone of a Beach & Mellow Yellow are the biggest hostels in Rio, and usually busy no matter what the season if you want to meet people.
They are both fairly similar - lots of different sized rooms ranging from doubles to 32 bed dorms! My advice is to go for a dorm in the middle sizes. The largest dorm room in SoaB gets really hot and in the bigger dorms in MY, I heard reports of bed bugs.
In SoaB all the rooms are named after beaches in Brazil. Macumba is the biggest room with most beds. Itacoatiara has a fan and no air con and is next to the breakfast lift which is noisy if you are a light sleeper. Grumari is pretty good, as although it has lots of beds which makes it cheaper, it's in an L shape, so it's almost like 2 rooms in one. Prainha only has 6 beds, and a balcony too - nice room. There's also a female only dorm (Piratininga) and ohers too. (I stayed in most of them, as I lived there for 5 months while working in the rooftop bar in case you're wondering how I know this!)
Both hostels have hot tubs - the one in MY is on the ground floor, the one in SoaB is in the rooftop bar. You'll meet lots of independent travellers in either :-) At busy times you will have to book in advance though.
Sampling local food - look out for the restaurants that advertise food by the kilo! You get your plate, help yourself, they weigh it, and you pay whatever it weighs! It's great! The most common Brazilian cuisine is rice & beans - together. You'll see this in any of the kilo restaurants - the beans are black in a black sauce. Add to rice, and choose your meat!
Of course there are also the chusscarias - the Brazilian BBQ restaurants if you like meat. There's one near the SoaB hostel (if you ask reception they'll give you directions) I only went once to experience it, and it was good but I lived on rice & beans for 5 months, as I loved it and it was free as part of my working deal most of the time.
I'm so jealous you're going to Rio!!