You were quicker to get signed up than me 
Is there any chance you remember the date when you received your card & activated it in the app? This is usually a couple of days after you transferred the initial £100 deposit, as the cards are then sent 1st class. You will have received your 1st ‘login to Monzo’ with a magic link on that day. And you might have tweeted your user number or made your first transaction around that time…
If so, could you please share it in this post -
We’re tracking Monzo’s growth here 