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 travel in Iran
A user suggested this hostel in Tehran. At least in the photos it seems fairly good: