ICQ did support offline messages, from the beginning afaik, too. I had a 6-digit uin (485358 or something similar), until it got banned for running a bot (whoops).
To the sibling reply, I think AIM and ICQ did have interop on messages at some point, it was much later than when ICQ moved protocols to OSCAR and TOK though.
I can't remember, but I don't think MSN had offline messages. And I don't think ICQ lost offline messaging in the OSCAR transition, IIRC, ICQ moved to OSCAR with offline messages, then AIM got them, then AIM and ICQ could talk for a while (but all my ICQ contacts that I kept had moved to AIM or MSN by then anyway).
As I recall, originally, the ICQ client polled the server via UDP to see if it had any messages, and then you would do peer to peer for online messaging. But when you logged in, you'd get a cascade of the offline messages (uh, uh, uh, uh-oh)
Woah, ICQ had peer-to-peer? I thought it was quite centralized! Was that before the OSCAR migration?
I only remember Skype being "true" peer-to-peer, with your PC randomly becoming a presence/call relaying "supernode" if you had a publicly reachable IP and good connectivity. Different times!
Yeah, ICQ was peer to peer for online messaging as I recall in the say 97-99 timeframe. I think Yahoo was too. They'd fall back to server message passing, of course.
But this was just for messaging (and file transfer), not for presence/buddy list which was all server driven.
In that time frame, few had firewalls or NAT or two computers at the same location, so (server mediated) peer to peer just worked unless you were on a corporate network.
There were even some tools that showed you the IP, the "real status" and etc. of your contacts. The 9 year old me have felt like the greatest hacker of all time when using those tools :)
Circa 2000 or so, when AIM, MSN, Yahoo! and ICQ were all flourishing, Yahoo had already added offline messages. ICQ, I think also had them, though it was probably configurable, I recall the client having a half dozen screens of options. At that moment, neither MSN nor AIM had it yet. AIM eventually did add it, though I don't recall if it was added to AIM after ICQ de-merged from the AIM backend.
There was also Meebo, which allowed you to login to all of them via a web interface (which I believe none of the messengers had natively) without installing the respective clients!
Meebo was sick. I remember they never made any money though as an IM webapp, and eventually fully pivoted to some kind of on-page ad toolbar that site owners would add to their sites...for some reason.
It was MSN. I remember when MSN arrived late to the game, and managed to get users anyway, despite not having such an obvious feature, that the incumbent had.
ICQ did support offline messages, from the beginning afaik, too. I had a 6-digit uin (485358 or something similar), until it got banned for running a bot (whoops).
To the sibling reply, I think AIM and ICQ did have interop on messages at some point, it was much later than when ICQ moved protocols to OSCAR and TOK though.