What does odd letter mean?

[LizzyBethBaby :o)]

I was talking to someone on Facebook and asked them if we could text . They said no because of something about apps and how she wouldn't be able to chat to other people 0.o but she suggested we odd lettered each other. I asked her what she meant and she said "I mean not constancy writing to each other cos i know people can get real busy but when we have time and feel like writing or sending something interesting we can  but no pressure or time limit on how long it takes for a reply as we can still chat here" I didn't really understand what she meant. Help? I've never heard of 'odd letter'

[Paladin]

it refers to an occasional letter