There's a point when someone walks out of your life that you've got to be strong enough to let them go. If you ever meant anything to them, they'll turn around. If they keep walking, then they were never good enough for you anyway.
Do they get butterflies? Do they have you on their mind 24/7? Do they wait for you to come online for hours? Do they smile at random times at the thought of you? Do they miss you all the time? Do they think about the smallest things you say?