I missed the low UIN boat on ICQ because I was solidly into FreeBSD back then and the ICQ client was only on Windows. It also looked sketchy as hell with the crudely drawn flower icon and public domain sound effects. I didn't join for years and ended up with a barely 7 digit UIN.
The typing and receiving may be similar, but the approach is different.
All of the above (save perhaps write) allow two users to converse freely, over a network. Sounds like a messenger to me.