Do random characters in GTA 4 disappear?

This will probably end up being the last thing players of GTA4 will get to on their quest for 100% completion. Simply because you can't control when the icons appear. I believe they appear only when you are in their immediate vicinity, perhaps also at certain times of your phones clock. I have tried driving around the designated areas until they show up, but that doesn't usually work. I notice they will appear after completing some other task. But if you consider the meaning of the word "Random", you should expect to find them at any time. Try pausing, and checking the map as you drive around. You might see them just out of the range of the radar. As you play this game start to finish, I suggest you drop whatever you were doing at the moment you see a blue "Friend" icon, and complete it's mission. And keep track of the ones you complete. Good luck!