[cmrdporcupine]

mastodon.social is by far the largest, but the recommendation is generally to find the one that has interests & people on it you find interesting to start. you'll get content from people on mastodon.social anyways.

[_cenw]

mastodon.social is silenced (no federated timelines) on quite a few instances, simply because there's a lot of volume that's not very high quality.

[augustuspolius]

This looks great, definitely more active and offers a good variety of topics/profiles.

[cmrdporcupine]

I went with indieweb.social. I think some people from hackernews might find a vibe they like generally there.