Facebook has opened up to everyone, not just college students.
Facebook | Welcome to Facebook!
There are many forums out there for discussion of specific topics. You're likely to meet someone in a specialized forum who shares your interests. Another good source is Yahoo! Groups; sign up for a Yahoo! Account, then browse Groups to find ones that appeal to you:
Yahoo! Groups - Join or create groups, clubs, forums & communities
Finally, consider becoming a regular participant on blogs that interest you. You can meet people and get to know them through regular interchanges on blogs. Google provides a page for searching for blog postings by topic that you might find useful:
Google Blog Search
Vasily