What I was told is that the content should be related to Hive, so avoid anything too personal and talk about something you know about Hive, for example HBD, HP stake, the qualities of Hive (censorship resistant, decentralized, etc.), its dapps (it can be games, frontends like peakd, ecency, the possibility of having NFTs too, etc.) so anything related to Hive.

X is the most commonly used, but it can be from other social networks as well. Some people post links from Instagram, but it can be from other networks without any problems. Promoting Hive is what matters.