BuddyPress - WordPress social networking in a box.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Elgg - Opensource social network engine
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Influitive - Influitive helps companies increase customer lifetime value and accelerate sales with the power of their own customer advocates. Inspire your customers to stay longer, share their experiences, and attract new business.