I have made a list of alternatives for some popular online services. This is not exhaustive, of course (there are other sites that do that well). These are just the ones that I have found most common and useful.
There are also a number of other open source tools worth mentioning.
- BookWyrm (Goodreads alternative)
- Codeberg (GitHub alternative)
- Flockingbird (LinkedIn alternative)
- Mobilizon (Facebook Events/Groups/Pages)
- Wikiless (Wikipedia front-end)