Open source is a good thing, but it's not enough by itself.
Reddit was open source until 2017, then they decided to go proprietary:
https://www.reddit.com/r/changelog/comments/6xfyfg/an_update_on_the_state_of_the_redditreddit_and/
You need things to be FOSS and federated and spread out on lots of instances in order to be relatively safe.