JSoup UserAgent, how to set it right?

You might try setting the referrer header as well:

doc = Jsoup.connect("https://www.facebook.com/")
      .userAgent("Mozilla/5.0 (Windows; U; WindowsNT 5.1; en-US; rv1.8.1.6) Gecko/20070725 Firefox/2.0.0.6")
      .referrer("http://www.google.com")
      .get();