Boyfriend always "Likes" this girl's facebook pictures?

There are these twin girls who went to school with us back in highschool anf they were GORGEOUS. They now live out of state, but my boyfriend is alway looking at their facebook pics and liking every single picture that shows their cute smiling faces together. I didnt care at first, but its been going on for like a year and every time they post a new pic he clicks "like" and its just getting annoying. Is thi anything I should be worried about? We've been going out for 3 and a half years.