EDIT: I'm stupid. theymos seems to be right. It's Adblock Plus.
Attentive users may have noticed that a new CSS rule has been added to the site, causing all cex.io links in signatures (and maybe posts, I have not tested) to be hidden:
::content a[href^="https://cex.io/"] {display:none;}
Would an admin mind explaining the rationalee for this. In addition, it should be noted that this can be bypassed simply by changing the capitalisation of "cex.io".