I've created several MVPs for startup ideas, so I understand how to pair large ideas down into the most essential proof of concept and build it.

What's the main value add of your new social network? If the main value add is that it's a community specifically for a certain group of people, I would start by actually creating a community and getting those people involved in the community. If you can show that people actually use and value a community of specific people, that is way more compelling than just email signups of people that are interested in possibly leveraging a community that doesn't exist yet.

If I were you, I would use something like https://clubhouse.cc or Slack.com or Google Groups or Facebook groups to actually create a community and see whether or not it's a value add to your targeted users. Then when you prove that there are people leveraging the community and engaging, it's an easy sell to an investor that you just need money to build a custom app for your community, rather than a shot in the dark based on email signups.