Dear Thomas,
Thank you for your suggestion. Type of communications between fellows is more informal and we don't need to have a history of them; this is the reason the program is relying on tools like Facebook and WhatsApp that many people are already using it and are familiar enough with.
We also need to keep in mind that many fellows in each round are newcomers that don't know enough about the communication mechanisms within the ICANN; so it would be hard to make them use some alternatives.
Both new fellows and alumni have separate mailing lists and this is the formal channel for the fellows and staff to communicate.