An article showing some user behavior intelligence @ facebook, It looks like a craze of facebook, i’m personally not impressed with it. People used to say similar comments @ microsoft then google and now @ facebook.
One more thing … facebook is using ecelerity as MTA (as in email content header of facebook emails)
Received: from [10.18.255.175] ([10.18.255.175:40036]) by notification+yy0cfcf9>) (ecelerity 184.108.40.206 r(34067)) with ECSTREAM id B8/DE-10283-AFF5E5B4; Mon, 25 Jan 2010 19:22:34 -0800 X-Facebook: from zuckmail ([MjAyLjY5LjYyLjM3]) by www.facebook.com with HTTP (ZuckMail);
Previously, i’ve heard of Port25’s powerMTA. (thanks to ashar bhai … our chief architect)