l.facebook.com and lm.facebook.com are known as Link Shim referrals. Their presence in Google Analytics indicates that your website has been positively verified by the Link Shim system. Every time a link is clicked on the platform, the system checks the URL against a massive internal database of malicious sites. If the system detects that a link is harmful, it redirects the user to a full-screen warning page, allowing them to safely return to Facebook.

• What is facebook.com / referral in Google Analytics?
  Facebook.com represents traffic coming exclusively from Facebook accessed via desktop devices.
• What is m.facebook.com / referral in Google Analytics?
  M.facebook.com shows traffic originating only from mobile devices.
• What is l.facebook.com / referral in Google Analytics?
  L.facebook.com is a Link Shim referral specifically for desktop devices.
• What is lm.facebook.com / referral in Google Analytics?
  Lm.facebook.com is also a Link Shim referral, serving the exact same purpose as l.facebook.com, but it indicates mobile traffic.