InterPals - InterPals is a friendly community of over 5 million friends, language learners, travelers and penpals.
BuddyPress - WordPress social networking in a box.
SLOWLY - Make pen pals from your smartphone
Elgg - Opensource social network engine
Twitter - Connect with your friends and other fascinating people. Get in-the-moment updates on the things that interest you. And watch events unfold, in real time, from every angle.
HumHub - HumHub is a free social network software and framework built to give you the tools to make teamwork easy and successful.