On every proposal there is a link to a PNG file (via "Share This" button, see screenshot by @BlasSoriano), which may be used to advertise the proposal.
I always assumed these files update themselves, but at the moment the file for the Latin Language proposal shows "90 followers", even though it should show "51% committed".
I just saw this ad on Academia.SE and it showed the correct "51% committed", so I'm not sure if it only shows "90 followers" to me (as a follower/committer of the proposal), or whether the ad really hasn't been updated since January 2015.
In any case, there doesn't seem to be a way for a normal user like myself to get his/her hands on an up-to-date ad for an old proposal.