Act 6 of Homestuck began on November 11th, 2011 – that is, 11/11/11. Act 6 is also set on this date, which is written in this universe as 11.11.11 and called 2X3PRONG DAY (whatever that means).
It's fitting that this should be the arc number for act 6, as it consists of six 1's.
Although almost certainly a coincidence, Karkat's first pesterlog – and in fact, the very first pesterlog with a troll – occurred in an update released on 11/11/09. The update on 11/11/10 had anotherlog with Jade and Karkat.
Gamzee's potions were all priced at 420 boondollars. 4/20 is the 111th day in a leap year.
Furthering their connection to the number, Feferi and Meenah both employ emoticons with 38s (representing their crowns and goggles) which equals 11 when 3 is added to 8.
Eridan's affiliated number, 311, obviously represents three 11s, just like his fellow sea dweller.
One of the pool ball combinations for Lrd English's eyes is 1 and 11, imitating one of Quarters's quarters, which likewise has the number "1" one side and "11" on the other.
Calliope and Caliborn sometimes multiply letters and punctuation by 11.
Sometimes they even multiply letters so that the total number in the word is 11