It was definitely an ad[0]. But the Reddit subreddit ads I think are excluded from Adblock Plus if you have it set to allow "unobtrusive" ads (which is the default)
Not the button I'm talking about[0]. There must have been ads too, but there was a generic non-ad "Did you press the button" button on the front page in the days following Apr 1[1].
[0] http://www.reddit.com/r/ads/comments/3181or/did_you_press_th...