Understand each other well.
Be on the same wavelength.
Find the same things funny.
Have the same sense of humour.
Stay in contact.
Keep in touch.
Not stay in contact.
Lose touch.
Know somebody better.
Get to know somebody.
Stop being friends.
Fall out.
Make friends.
Fall in.
Like/enjoy the same things.
Have a lot in common.
A person one knows slightly, but who is not a close friend.
An acquaintance.
Like to be with somebody.
Enjoy somebody's company.