Thing is how do you know how well one person knows another? I tend to use other people's first name but usually it's because I've met them or exchanged PMs back and forth. If someone's name is Mark but their handle is 98superchargedfastmustanggt typing Mark is much easier.
I raced FFW and NMRA for 4 years and met a lot of people from all kinds of Mustang boards plus all the locals I've meet at Moroso/PBIR over the past 17 years. At times we'll go back and forth using first names. Maybe it's strange to some but it's normal to me.