How did the largest consumer apps get their first 1000 users?

How did Pinterest, Producthunt, Musically, Instagram, Whatsapp, etc get their first 1000 users? Quick thread on their growth hacks :)
Would love to know some from the last year or two. Growth tactics are so different now!
I'd recommend many of Andrew Chen's resources here:!
Here's an interesting post on the topic (including Pinterest)