Actually contacting them is less common, but certainly not unheard of. Whenever the subject is broached, it's almost invariably presented as a bad idea. This has actually been the theme of a few movies, where it always seems to backfire in one way or another.
But curiosity is hard to extinguish. What is your ex doing these days? How has he or she aged? What is the ex's love life like? Facebook can answer some of these questions, but theres no substitute for actual contact.
Getting in touch often has to do with wanting to show off. If you're doing better than the last time you saw each other, you want the ex to know that. The desire to boast can never be overestimated.
And there's always the hope that the flame might be rekindled. Which makes the potential for awkwardness immense, given that that hope is likely to be one-sided.
In a way seeing old friends is the closest we can come to time travel.