Why not just sidestep the whole thing by saying “I tried to find you on Facebook, but I couldn’t. Do you use a different name there?” Plenty of people use a variation on their name or even something made up. Or if even that seems a bit too forward, you can just say “Hey, are you on Facebook?” Then it’s on them to say yes or no and to tell you how to find them. Basically, just approach it the way you would if you didn’t suspect them of being transgender. They’re just a person you want to be friends with on Facebook but couldn’t find their profile.