There's a memory leak that only got fixed in 0.10.22 I believe. Erin Hammer and the whole Node Black Friday team really publicized it when it was biting them[1]. I'm not sure if the leak was present in 0.6 and given that you rely on older modules the cost/benefit of switching is probably too high.
You're the first person I've heard that was able to run vanilla Socket.io at such scale. I know Trello and some other shops use Socket.io, but they've forked & changed it to something that looks nothing like the original[2].