I don’t know the uptime figures for X/Facebook or other platforms.
However, HIVE’s uptime can be estimated using head_block_number / current_aslot, which comes out to roughly 99.75%.
In practice, occasional missed blocks by witnesses don’t affect HIVE’s usability at all, so I believe the blockchain’s real-world availability can easily reach 99.99%. In fact, as far as I can remember, since the fork from STEEM, the HIVE blockchain has never experienced a full stall.