Right now if you share an "R-18" image via the twitter button, you get this:
#r-18 toujou nozomi (love live! school idol project) drawn by frapowa http://danbooru.donmai.us/posts/1691527#.U3vQWUH7ORM.twitter
This is improper twitter syntax, the '-' after the 'r' Breaks the hashtag, so only the "r" works.
Proper syntax would be:
#r_18 toujou nozomi (love live! school idol project) drawn by frapowa http://danbooru.donmai.us/posts/1691527#.U3vQWUH7ORM.twitter
With an underscore instead of a dash.