IndieWeb
I participate in the IndieWeb, a collection of personal websites meshed together into something resembling a social network.
Why?
Interesting question. A lot of IndieWeb enthusiasts will tell you it's about keeping control of your own data. This is true for me to some extent - I want to keep all my blog entries in a form that will be relevant in 20 years time and safe on my disk, though maybe that's more about using plain text than about controlling your data.
Photos too; I like keeping those under my control.
For things like notes or likes, it's less important, but still kinda neat.
How?
Basically microformats. They are effectively the API of the IndieWeb.