to be fair you're comparing Steem which is POS to the other coins which are POW. POS is always faster and more scalable than POW, and thats why ethereum will switch to POS (Casper) at some point. even the soon to come Bitcoin Lightening Network is essentially POS.
if you had included other POS coins in the comparison doubt the difference would be so marked.
a fully scalable blockchain for socialmedia I don't think it exists yet. in Steem I believe only text goes on the block chain but pics and videos need to be hosted elsewhere . it's the best that can be done for now.