Referrers: Facebook

Hello,

who knows what the following Facebook referrers mean:
l.facebook.com
m.facebook.com
lm.facebook.com

Are these specific posts, the mobi version or for what do these letters stand for?
Thank you very much.

There are already some discussions about that on the web. Have a look at http://www.thelasthurdle.co.uk/new-facebook-referrals-in-google-analytics/ or try to google something elseā€¦