For Mastodon: https://joinmastodon.org/

Nextcloud is included because it's part of the fediverse: https://nextcloud.com/blog/federation-a-foundational-concept-for-digital-sovereignty/

Mastodon is fairly popular and well supported if you like Twitter-like micro-blogging. Some Lemmy instances actually federate with Mastodon instances, so you can post to Lemmy communities from Mastodon, and vice versa.

I do have a peertube account I use to host videos sometimes, but otherwise I don't personally have any recommendations beyond thinking Mastodon is a relatively mature and popular platform.