Not sure whether Bungie is able to determine the source composition of fireteams, but I'm assuming the statistics would show a majority of fireteams are either a mixture of real life friends and people found on external websites, or entirely from external websites (either randoms or friends originally found/made on them). This poll will likely skew toward the real life friends options based on casual players being less likely to use these forums and more likely to not participate in multiplayer activities that don't have in game matching.