Referral or affiliate links fall under the "spam" category. Any effort by a member to profit by a posting on POTN, outside of a Marketplace For Sale listing, is spam.
The Zenfolio and SmugMug referral threads are formally-sanctioned exceptions of long standing to that "No Referrals" rule.
Zenfolio referral code thread: http://photography-on-the.net/forum/showthread.php?t=281592
SmugMug referral code thread: http://photography-on-the.net .../showthread.php?t=1001854