If you love someone you can't tell them on Facebook. If he is a crush and you have never dated then you shouldn't tell him at all. You have no idea if he feels the same way. Instead, start out slow and suggest you two hang out together and try to build a relationship from there. If you told him you loved him and he didn't feel the same it would really hurt to be rejected. Don't do that to yourself. Give him a chance to get to know you and decide for himself if he cares for you. Don't scare him away by acting to soon.
umm you dont. you can't tell someone who is only a crush*implying you arent dating, arent really good friends, dont know each other well* that you love them, because you dont. you can't love someone you dont know that well...also its really lame to say something important like that for the first time on FB