I've been online since 1993, on the web since '95... Depending on which aspect of user-generated content we're talking about, AOL might have beat GeoCities to the punch. By 1995 almost everybody online was also on AOL, and the majority of those frequently updated a "Profile" with Facebook-like content.

If the issue is freely-created user content like websites, then I'm not so sure we really have anything like that in popular use anymore. Most people just share scattered crumbs -- a photo here, a post there -- as they occur now; almost nobody creates a site of information or tales related to an interest. If anything is the precursor to Web 2.0, it's texting cameraphones.