Scalar feels like a better scholarly sandbox.  It has the advantage of allowing length of
thought (unlike twitter) as well as an improved meta / path ability than
BB.  Social Book was valuable for
annotating and discussing an existing, fixed, work, but was not the place for
the creation of a new piece. Social Book felt like a seminar  --  scholarly
discussion and creation of  understanding
through interchange.  Scalar feels like
the production of new meaning  --  the writing one might return to and revise
and see as a complete record of thought. 
A case could be built in Scalar. 

While one writes in Social Book, it is as a reader
responding.  In Scalar, the opportunity
is there to be the writer as well as responder. 
There is the opportunity for collaboration.   While the author may respond in Social Book, it is less likely than in Scalar where
the work itself is still being created.