Creating accounts on the various social media sites is completely free, as is sharing your content with followers on those sites.
You don't have to be on every single social media channel for most website owners that would be overwhelming. Take some time to research where your audience is most likely to be and how they use the different sites.
If your audience generally prefers written content over video, then you may not need to be on YouTube. A photography website should definitely be on Instagram, but one that's light on images may do better on Facebook and Twitter.