Social buttons are now widespread in social media apps. They are used to assign weight to user content and trigger user engagement. They come in different shapes (e.g., thumb in Facebook, arrows in Reddit or StackOverflow, plus one in Google+) but very little is known about the influence of the shape on user behaviour. This paper addresses this issue by presenting results of a controlled randomized experiment with 173 users. The results suggest that thumbs up / thumbs down icons are significantly more engaging than the plus one / minus one icons. At the same time, the result shows that type of the icon used has no significant influence on the direction of the vote.