It is a smiley known as the "Itachi smiley" because of the lines beside the underscore which is the mouth. It is also commonly used by rabid Itachi fans.
Other Itachi smileys include the entire selection of anime smileys just with / \ beside the mouth.
itachi_lovr101: omg, i watched a new episode of naruto today and Itachi-kun was in it! ^/_\^
me: .....i hear Itachi kills the rabid fans who use his face as an internet smiley