It's not just communication but the ability to easily keep track of where people are in the world and to coordinate meetings and shared holidays. Facebook is particularly good for keeping in touch with old friends and acquaintances because of the contextual information it supplies. I saw recently that a friend I haven't seen in years is spending a month in Thailand and I arranged to change my travel plans so that I would be passing through during the same period. We weren't close enough friends that we would still be in touch if it wasn't for those kinds of coincidental connections.

I also meet a lot of new people through couchsurfing.org and various expat sites. Couchsurfing is particularly interesting because its goal is explicitly to create friendships between people of different cultures. I would probably be much more lonely when traveling without it.