My Girlfriend keeps checking her exes profile on facebook?

Ive been with my girlfriend for 5 months she's absolutely amazing, she recently saw a guy she dated when she was out, didnt say hi or anything, although they have no contact whatsoever and are not even friends on facebook , she keeps going to his profile to see pictures of him and his new gf, she's checked maybe 4 times in 3 weeks, is this normal?