HCS is based on High Skill, EXP Effectivness (win rate), Ranked K/D ratio and Ranked Games played. You also get small bonuses for having a lot of headshots and for getting the HaloCharts awards. The only way that social games help is in terms of EXP Effectiveness. A big chunk of the score is based on your Ranked K/D ratio over the number of games you've played. Obviously a ranked K/D of 5 with 100 games isn't worth as much as a 2 K/D with 1000 games.
So keep all of that in mind when you're playing if you want to improve your HCS.
Also, High Skill 50 gives a small bonus to your HCS.
HaloCharts.com • View topic - How HaloCharts Ranks Players
http://halocharts.com/forum/viewtopic.php?f=3&t=29