I need to know:

   * The percentage and absolute number of people among the the
American population who send personal letters (snail-mail) to their
friends and relatives.
   * How this data is affected by age and sex.

If you can tell me how many different personal mail correspondents
has, on average, someone who sends letters, expect a tip. I'm also
interested in how many of those are family and how many are friends.

Current data: I already know this research data
http://www.comm.ucsb.edu/faculty/rrice/syntopia.htm