The more I look at Facebook Groups and the long list of functions that come with it and the more I find similarities between Facebook Groups and eGroups, a service that started life as an email list management service, that was later acquired by Yahoo.
eGroups was launched in 1997 and was acquired by Yahoo in August 2000 for $413 million with a user base of 18 million users, becoming Yahoo! Groups in the process.
While 18 million users is a small number, it needs to be put in context; Hotmail had 30 million users in February 1999 and Google wasn't even on the radar. Like eGroups, Facebook Groups allows users to bring together privately people with a common interest without sharing data with the rest of the ecosystem. eGroups used your email, Facebook uses your Facebook ID; once you create the group, you can collaborate on documents, edit wiki, run a discussion board, have a calendar, have a chat room (remember IRC?) etc.
All these were already present in eGroups and another big community-building website that disappeared in the last internet bubble, eCircles.com. But Facebook is both sleeker, faster and much more stable.
The next step would be for Facebook to launch a Professional edition (like Linkedin) using groups as a formidable enterprise tool; one that could also challenge Microsoft's Sharepoint, Salesforce and Google's Apps.