last night there were more than 4500 concurrent visitors and the site was usable
You may be surprised to know that at its peak it reached over 11,000!!
Load-limiting was put in place to keep things running, as there was a short period when the database wasn't keeping up (dbase errors). Basically, priority was given to registered users over guests for accessing the forums. After the load-limiting was done the site was coping admirably with almost 9,500 concurrent users.
So yes, hats off to the folks at IB who are running the back-end servers and networks.
SD