Network effects aren't a big deal when it comes to messaging. There was a time when people thought iPhone wouldn't be able to overcome Blackberry because everyone was on BBM. In the last couple decades we've seen people go from ICQ to AIM/Yahoo/MSN to Google Talk to Skype to Facebook Messenger to BBM to Whatsapp/iMessage/Instagram, with dozens of smaller options like Kik, Viber, Line, Signal, Telegram all hanging around. It doesn't take much to cause another shift in the space.

It's a problem but not insurmountable. Otherwise we'd all still be using ICQ/AIM/MSN Messenger/Skype/etc.

I don't find there is much network effect for one on one messaging. I have to use a few different apps to talk to all my friends, it's not a big deal to switch to/from Signal or Whatsapp. With groups it's more effort.

It's easy to have multiple chat apps in parallel though, each with their own network.

Ads will make more people Signal-curious, or even drive people back to text messages. The average person who switches will convince a non-zero number of their contacts to come with them. The shift will start gradually. Think of Skype, which at one point everyone I knew was on. That network didn't protect them from being replaced by competitors.

People are also increasingly worried about retaliation from the government for their supposedly free speech, which has already driven a few people I know to secure alternatives that aren't operated by Trump allies.