Numa is a new social network built on Ethereum. Open source and federated, it can also talk to other such networks, including last year's competitor-to-Twitter-for-around-a-week, Mastodon.
Although it's built on Ethereum, the developers are keen to point out that you don't need, or need to understand, cryptocurrencies to use it:
The only costs you pay are transaction fees for interacting with Ethereum. Numa doesn't charge any fee for tipping, signing up, or posting message. We've engineered our smart contract to be as cheap as possible. We allow you to batch all of your updates - including signing up, posting messages, following people, and more, into one cheap transaction. Each transaction only costs $0.01-0.10 USD, so you can post messages for just a fraction of a cent! You usually don't have to worry about your transactions getting confirmed quickly, because we keep track of and display pending messages.
Because we batch transactions, you don't even have to spend anything to sign up. You can start using Numa without owning any Ethereum. Only when you're ready to post your data on Ethereum do you have to spend anything.
There's no mobile app at the moment; it's all browser-based, but it's an interesting proposition as an alternative to the profit-making walled gardens out there. This won't replace them, but could augment them in a peaceful coexistence, as many services have already done.