[Firebird]

I traveled Japan alone for 1 year and my experience is following:
As long as you are in big cities and stay in Hostels you will easily meet people. Most of them will be foreigners. The smaller the town, the fewer foreigners and the more Japanese you will find. Japanese usually stayed in Hostels wich belong to the international Youth Hostel Organization. Foreigners will go to the new Hostels wich are normally alot better, cheaper and more people speak english.

Japan has alot of hostels in bigger Towns, smaller places will only have Minshukus or Ryokan wich usually charge a little higher fee for a single room than youd pay for a dorm room in a city.

If you stay in Saunas or Capsulehotels to save money, you might have a hard time to meet people.

Anyway, you said youll stay in Hostels and in Tokyo + on other big city so id say dont worry, you will meet people!

If you only travel a long distance once (like Tokyo-Kyoto) you might want to take a nightbus instead of buying a railpass. The railpass gives you an amazing value if you travel around alot, but if you plan 1 week Tokyo and 1 week Osaka than buses will be alot cheaper and even a single Shinkansen Ticket might be cheaper. If you travel 3 or more times over big distance than get the pass!