Birthdays are easy. The random Wednesday check-in is where friendships really live.

Anybody can show up for the scheduled occasions. A birthday is in the calendar, everybody is prompted, and turning up proves only that you can read a notification. The unprompted midweek message is different, because nothing triggered it except you thinking about somebody for a second and acting on it. That is the actual substance of a friendship, and it is almost entirely made of small ordinary contact.

Send three this week with no occasion attached. A photo of something they would find funny counts. Keep the bar low enough that you will actually do it, because frequency beats quality here by a long way.