If you filter your Referrals report for facebook, the “l” (that’s l as in lemon) referrals were directed through Link Shim, which Facebook uses to protect clickers who may be unknowingly accessing malicious sites. If your site is legitimate, Link Shim is no problem, but the referrer will still appear as the l version.
This apparently represents an improvement for analytics, because the current Link Shim versions previously registered in Google Analytics without accurate source attribution, so you could not even tell that the visits originated from Facebook.
To sum up the four combinations of desktop/smartphone and Link Shim:
- facebook.com – desktop or tablet, link not directed through Link Shim
- l.facebook.com – desktop or tablet, link directed through Link Shim
- m.facebook.com – smartphone, link not directed through Link Shim
- lm.facebook.com – smartphone, link directed through Link Shim
For more information on Link Shim referrals in Google Analytics, check out this great post by Jules White.