I take your point about a demo being a demo... as they say the proof of the pudding will be in the eating.
I think first mover advantage will be crucial. Steemit has a chance to lock down the market before some of these other projects get off the ground. I do think they are slightly different propositions. It will be interesting to see if Synereo can scale.
Certainly - there's scope for different types of social networks. Steemit has got the Reddit-style platform covered. Synereo does seem to be a slightly different twist. What worries me though that they are several months away from launching a website. Their whitepaper is dated all the way back to March 2015, and they are only now showing a demo. A pretty similar story with Akasha - I have been hearing about for many months now, yet they have nothing to show for it.
The idea of a blockchain based social network is a popular one that has been around for years, but it's only Steem that have gone ahead and actually implemented and executed it. It remains to be seen how it will scale in the long term, of course.