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The point of SBD is that it worth a US-$. If BTC=$500 then SBD=.002 BTC.
It remains to be seen how well the peg holds. Since SBD pay 10% annual interest you can expect them to trade above 1 USD.

I discovered steem, I would like your opinion on the network steem the support is reliable? are there any person who REALLY boss on the evolution of steem?

Not sure I understand your question.
On the technical side I have no doubt that the steem network is reliable.
On the economic side I don't see how this can work out long-term.