...all readers and writers have experience with even smaller containers for good writing: a journal entry, a haiku, a telegram, an epitaph for a gravestone, a love message on one of those candy valentine hearts, the chorus of a song. Back in the day, headline writers counted available spaces per line, and poets still count syllables. Writers on Twitter and Facebook just happen to count characters (or to have characters counted for them by The Machine.)
This is a good thing. Any metric of length forces a discipline on the writer.