If you break up the modules of Facebook you use everyday, there are some effective alternates for each of those:
Single-Sign-on (SSO): Facebook is quite effective when it comes to letting you in on streaming or health app sites. But those sites also have their own login or sign in via Google. You should setup unique password for each site and then use an effective password manager.
Groups/ Messenger: This would be difficult to replace if you have a very popular group or community. While Google Communities come as second choice, you will still end up playing into the hands of Google! I’d rather recommend you go with Discord for channelised group conversations and Signal or Whispeer (not WhatsApp-as it now belongs to FB) for a secure private messenger experience.
Marketplace: IMHO this is a space that FB has not conquered yet. I have not seen FB to be a popular site to buy/ sell stuff (privately). This is because most folks would rather prefer local community sites that are more prominent in their country/ city.