Ask HN: How important is the absence of forced user account creation to growth?

It seems like several of the world’s fastest-growing websites/apps grew so quickly because they a) offered value to users without b) forcing them to create accounts.

Here are four examples:

1) Google – Users can get search results and Google can monetize them, all without accounts.

2) Reddit – Much of reddit’s value is giving users interesting information, and displaying a conversation around it. Accounts exist, but they’re not necessary.

3) WhatsApp – Users technically have accounts, but they never have to sign up and create a username/password. The phone number is the auth.

4) Wikipedia – Search the world’s largest encyclopedia. No accounts.

Do you believe these services would have taken over their respective markets if users were forced to create accounts? Any ideas for a resource you wish existed without the need for an account?