I might have been 10 minutes too young for ICQ. I think that's what the college kids were playing with when I was in high school. For my cohort it was the big three: MSN, Yahoo! and AIM. You probably had all three installed on your computer and probably all running at once. They're probably why my entire generation can touch type. Vital tool for teenage social life at the turn of the century.
This was Microsoft's era, too. The main reason Apple survived the 90's was because Microsoft invested in them to counter anti-trust allegations. They paid Apple to keep existing so they couldn't be called a monopoly. Internet Explorer was the web browser, any others in use were a rounding error. No one had a Mac, a few people were still clinging to their Amigas. THE platform for personal/home computing and internet access was a Pentium PC with Windows ME or XP, which came with MSN Messenger out of the box.
Two things happened nearly simultaneously: Facebook Messenger and the iPhone. Graduating high school in 2005, your freshman year of college you probably started hearing about the cool new site that's kinda like MySpace except it's only for college kids. By your junior year all your new college friends were on Facebook and all your old high school friends that never logged on let alone talk to you were on MSN. And if you graduated in 2005, your junior year was in 2007, the year the iPhone was launched. MSN Messenger had been present as baked in "functions" of certain media phones at the time, but I don't think they ever made it to the App Store or even the Play Store on Android. Facebook was fast to adopt mobile apps, and for awhile there it was the one messenger service that interoperated between desktop on a web browser and smart phones across platforms. SMS didn't run on the desktop, iMessage is Apple-only, AIM, MSN and Yahoo were nowhere to be found and Telegram, Signal, Discord etc. weren't around yet. So everyone standardized on Facebook Messenger.
Meanwhile, Microsoft bought and ruined Skype.