I usually just make something up. So do a lot of people. Goes to show how useful this information is to them for "tailoring and marketing to meet the needs of their customers" or whatever it is they purport to be doing.
I'm sick of this trend towards being asked for personal information in order to make a purchase. Can't I just buy something, like in the old days? I went into the Canadian Chain That Sells Tires (as well as housewares, hardware, sports equipment, barbeques, etc) because I was on the road with a 5 hour drive ahead of me and one of my tires was losing pressure faster than I was comfortable with. I just wanted someone to LOOK at it, while I waited. Before I could even talk to the person in the automotive section, I was asked for my name (fine) and email address. I asked why they needed my email address, and asked if I was going to get a load of junk mail. The answer, "Oh no, nothing like that, we just like to send out notices of our special promotions, stuff like that." Well I don't know how they define junk mail, but I define it as exactly that. So I said absolutely not. They were OK with that, though a little taken aback. It just irritated me to be asked a load of questions just to have someone check my tire. (Turned out to be a leaky valve or something).