Just to share a bit more of the backstory 
- Tom’s said that they were only expecting to sign up a few thousand users at the most, before the launch of the ‘full bank’ & current accounts. And my guess is that this is what Starling (the most similar ‘challenger’ bank to Monzo in terms of the business model, who have taken a very secretive approach, has ended up with).
But a combination of being open (community, blog posts, office open days, presentations), the hot coral card, p2p payments creating a network effect, a strong fintech scene, using the right tone in their content / social media etc. etc. etc. has meant that there’s been far more interest than expected which is how Monzo’s ended up with so many more users.
So following on from Naji’s comment, this has led to Monzo proactively stepping up their sales / marketing efforts, culminating in the week long Old Street shopping event / marketing campaign at the end of this year -
& also raising more money from investors, to help pay for the increased operating costs & I would guess to also fund more marketing.
The plan for next year is for Monzo to double the size of their team but their target will be to increase the number of Monzo users at least several times over. Obviously rapid growth in terms of users is one of the key signals that investors are looking for 