If you are interested in meeting people I would definitely suggest Couchsurfing. As an Iranian I can assure you they'd love to provide host, specially to foreigners.
You can find many CouchSurfers in big cities.
Here is just a sample search for Tehran.
Unfortunately, in Iran there are not (yet) many good hostels as you can find in Europe. And you shouldn't expect to see an international environment in the hostels there either. Maybe in close future but not yet.
Also TripAdvisor has a great database of comments for hotels and restaurants. An an example I give you a list of restaurants and hotels in Tehran sorted by their rankings (A restaurant is always a good place to meet/communicate with Iranians):
Enjoy your trip to Iran
Seems like there have been changes since my original answer and recently more qualified hostels have been giving service to tourists! I'll gradually add other suggested hostels here (list is sorted by date).
- Seven Hostels:
Seems fairly good (at least in the photos!).
- Tehran Hostel:
Location: Tehran (obviously!?) - GPS coordinates: 35.709248, 51.476587
Close to the Sabalan metro station and BRT bus line.