> I don't know a process for becoming the one with the largest userbase.

I was at WhatsApp from 2014 - 2019. Growing a large userbase from scratch doesn't happen by any one factor. You have to do a lot of things well. (and probably get lucky)

a) potential users need a compelling reason to join. Messaging at data rates was significant, but not in the US were many people had large messaging allowances. Works better than SMS/MMS was compelling for some.

b) existing users need to be satisfied enough to stay: service has to work consistently, client has to work, etc.

c) signup flow needs to work well. Doesn't matter if people want to use the app if they can't. You need to help users understand their phone number (or other identification). You need multiple methods of verification, because SMS doesn't always work. Giving someone a several digit code over the phone is a cognitive task for the user, and it's harder with disjointed speech generation, so you need to spend some time on that too. You need multiple providers because if you can't get verification codes to users, some of those people will give up and never come back. Since you have multiple providers, you need to figure out how to pick one based on current conditions which you also need to figure out how to track. Also --- you need some money, sending all these codes gets expensive. Phone numbers as ids is a blessing because "everyone has one" and you can use the system address book for contacts, but verification costs add up; usernames or email as id make contact discovery messy and a surprising amount of people in the developing world don't have an email address or don't know what it is.

d) users get new phones, a lot, you need to make it easy to move their account. Or they will likely drop your service when they get a new phone.

e) you need to be prepared for and handle large events. If some big news happens, people will want to talk about it. If some similar service has an outage, you will get more traffic --- if you also fall over, that's a lost opportunity.

f) things need to work well on the devices people actually have. Which might not be the ones you would prefer to use. Worldwide, most people don't have flagship phones. If you want a large number of users, having good experiences only on recent flagships is self limiting. Working well (or at least better than alternatives) on low end and older devices is a path towards addressing users that others miss.

There's probably more. Most of these require sustained consistent effort to deliver. It's not a one time thing. And it's not quick. Sustained consistent effort is easy enough as a one product start-up, but it's very hard as a big-corp.

Userbase can be a positive feedback loop: once you have enough users, that becomes its own reason to join ... and having no one to talk to is a reason to leave. There's not really a way to jump start it, unless you've already got a large user base somewhere else that you can use to seed your service.