Custom facter fact to report the primary user, determined by greatest total login time. The paths are for linux. Adjust as necessary.

primary_user.rb

# primary_user.rb
Facter.add("primary_user") do
  setcode do
    %x{if [ -f /usr/bin/ac ]; then /usr/bin/ac -p | /bin/sort -nk 2 | /bin/awk '/total/{print x};{x=$1}';fi}.chomp
  end
end

I love this idea. Thanks, Peter.

# Obfuscated with Ruby
# needs to split on 2 or more whitespace to handle user names with spaces
Facter.add("primary_user") do
  setcode do
    (`ac -p`.split(/\n/).map { |l| l.strip!.split(/\s{2,}/) }.sort_by { |a| a.last.to_f })[-2][0]
  end
end