How do you compete as a communication platform like a chat program or a social media.
Specifically, how do you get off the ground in the first place?

For a service where the main point is to talk to/see other people... it doesn't matter how many cool features you have if there is no one [you'd want] to talk to/see there, so you end up in this circle situation of "no users, so no one joins, so no users, so no one joins, so no users", unless a bunch of people decide to swtich to it in a short time and stick until it becomes the 'status quo'.

Also, the main goal of any communication service is to become the status quo, people of course use multiple services, but no one uses multiple all with the same relevancy, it can get unpractical to manage multiple friends lists and threads in different places, and it's also harder for someone to want use two services that are in the same area.

See this with things like Signal (to Whatsapp) or any attempt at an YouTube or Twitch alternative