webcal is just an unofficial URI scheme for accessing icalendar documents. You can get the calendar using http and parse it using one of several icalendar plugins:
require 'net/http'
uri = URI('http://www.facebook.com/ical/b.php?uid=13301632&key=asdgagaweg')
calendar = Net::HTTP.get(uri)
For alternative ways to perform the http get request: How make a HTTP request using Ruby on Rails?
Then parse the response using your favourite plug-in:
cal = icalendar.parse(calendar).first
vPim:
cal = Vpim::Icalendar.decode(calendar).first
cal = RiCal.parse_string(calendar).first