Ping is a simple way to exchange information with people you meet - a digital business card if you will. However this is just the foundations for a personal CRM - a tool that will help you keep on top of and organise your network.
Dex is a personal CRM that reminds anyone to keep in touch with people they would otherwise forget. Dex integrates with your calendar and social networking sites to send you actionable suggestions with no work required. Make staying in touch easy.
I coded the first version of Keep My Friends after getting one too many "I haven't heard from you in months" messages from my friends. Everyone is busy (including me), but I found it very easy with the app reminding me to keep in touch with important people in my life and maintaining Streaks of unbroken contact in the app.