GNU social is a social network for microblogging. It enables you to publish short notices including URLs and pictures. If you're interested in what someone has to say you can follow them. When you log in to your account you see a timeline containing all of the notices from people that you follow. You can have a conversation with another person by replying to each other's notices.
You may have seen this sort of thing before. GNU social is special for two main reasons: it's decentralised, and it's free software.
Being decentralised means that there is no single server that controls GNU social. Instead, many servers are run by different people around the world. These servers communicate with each other to form a federation. You can create an account on any one of them. Although the servers sometimes look different, ultimately it doesn't matter which one you choose—you're still part of the same network as everyone else.
If one server suffers an outage it's inconvenient for the people who have an account on that particular server. The rest of the network continues to operate as normal. This makes GNU social highly resilient. Censorship is difficult as servers can be located anywhere in the world.
Because GNU social is free software it's here to stay. A corporate social network might disappear or start running advertisements when the venture capital runs out. The GNU social code is available to everybody and there are many servers where it's free to create an account.
This is a social network that does what's best for the people who use it—not what makes the most money.