What could be limiting my Socket.IO concurrent connections on the same system?

I have a NodeJS client app trying to make 10,000+ new connections with a NodeJS server app using socket.IO. Both are running on the same box and the client is able to create 233 connections before the server just stops accepting additional connections. No error, just the client sitting waiting for the 234th connection to be accepted.

I thought it might be a max-files open limitation, but I already modified that limit:

cpu         unlimited      unlimited      
filesize    unlimited      unlimited      
data        unlimited      unlimited      
stack       8388608        67104768       
core        0              unlimited      
rss         unlimited      unlimited      
memlock     unlimited      unlimited      
maxproc     709            1064           
maxfiles    12000          12000

Here's how I'm connecting:

var socket = io.connect('http://localhost:3001', {'force new connection': true});

Any thoughts what might be limiting me?