Revolut have just tweeted that they’ve reached 700k users 
https://twitter.com/RevolutApp/status/884813497963565058
the link in the video is to this GIF
Edit 1
but to put that figure in context, it looks like they hit 600k users somewhere close to the end of March, based on these sources
1 (news story from Bloomberg - March 14, 2017)
Revolut Ltd. has almost 600,000 customers
2 (tweet from The Marketing Eye - 30 Mar 2017)
@RevolutApp has acquired 600,000 customers
3 (news story from Altfi - 19th April 2017)
Revolut now boasts over 600,000 users globally
So it’s taken them about 4 (edit - see JPiz’s note below) 3 1/2 months to gain another 100k users.
Whereas Monzo is on track to reach 300k users within the next week or so which means it’ll have grown by 100k users in just over 2 months…
Edit 2
Could someone please check my maths here?
If it took Revolut ~4 months to gain 100k users & assuming that a calendar month has 30 days, then they would need 4 x 30 x 3 = 360 days to gain another 300k users.
Instead there’s 120 days until the end of the year. So wouldn’t their user growth would need to speed up by 360 / 120 = 3x in order to reach 1 millions users by the end of the year? 