A decade and five

A social networking site with 100,000 groups, 1.5 million members, eventually acquired by Amazon, with features such as:

when a user entered travel plans into the calendar, the service would cross-reference the destination with the address book, as well as the user's contacts' travel plans; the site would then notify users when they would cross paths with their contacts.
All in 1996? Read the Wikipedia article about PlanetAll.