There's a point where "everyone for themselves!" doesn't quite cut it. The most obvious example is probably fraud. You could argue everyone must check whether a deal they get involved with is legit and accept the risks of being screwed. But that's not how it works since we must enforce a basic level of trust by law for society to function. A slightly less obvious but still rather well known example is addiction (drugs, gambling, etc).

The reason we elect politicians rather than deciding how to deal with big, social issues like that ourselves is because it's a full time job. Most people don't have the time and have no obligation to research "free-software based & decentralized social networks". They don't know about it and don't have the time or education to even know there's alternatives. We're so cynical about politics that it's hard to even trust politics to solve any of this since it's all so backwards and corrupt but I believe, because of the sheer impact it has on society, this is a political problem and needs to be solved through actual laws.