All the early startups I worked with used AIM or MSN Messenger. The main thing Slack did was 1) give each company their own private walled garden, 2) integratio

All the early startups I worked with used AIM or MSN Messenger. The main thing Slack did was 1) give each company their own private walled garden, 2) integrations/extension support. AOL or Microsoft could've owned this space in the 90's... if they thought about it.