Recently, My Random Guide was suspended due to a ToS infraction. Luckily, Britney was able to get her blog unsuspended. It seems that the reason for her suspension was due to her posting of referral links, of which, she would receive some monetary benefit.
I’ve said this many many many many many MANY times, but it seems like I still need to say it. Referral links are not allowed on WordPress.com blogs. Here is a link to a forum post that also help illustrate this point: http://en.forums.wordpress.com/topic/am-i-allowed-to-post-a-referral-link-on-wordpress?replies=1
Also, as someone points out, having a referral link appear on a Twitter widget will still count as a violation.
From this, it is safe to assume that using a shortlink to “mask” the referral link (like what a certain blogger is doing) doesn’t make it okay to post it. Also, it isn’t a violation to mention a site (but only WordPress themselves can verify this), such as InstaGC. Or even linking to them. Assuming that link isn’t a referral link.