Virality and network effects drive organic growth.
The best consumer companies incorporate both virality and network effect, which are different concepts but very closely related, in order to grow their user base organically.
Virality is the idea that one user using your product introduces it to other users somehow in the use of your product, whereas network effect is the idea that the product gets better the more nodes in the network exist.
I'll give you an example from Facebook's early days. So, when you took a picture on Facebook and uploaded it, you were prompted to tag your friends, even if they weren't already on Facebook. Those friends would get an email saying, "Hey, someone's tagged a photo of you; sign up to view the photo." That's a viral mechanic.
By uploading the photo and tagging your friends, you spread it to new users. A network effect comes from Metcalfe's Law: the value of the network is the square of the nodes in a network.
What that basically means is the more people who use this thing, the more valuable it gets. So, WhatsApp is a great example of this.