PostgreSQL is a good open source alternative for GIS and enterprise needs (Oracle, DB2, MSSQL). Unnerving is a tiny fraction of the Postgres community that acts as trolls.
I think you may have some interesting information to add, but your comment currently reads like aped talking points, with a dash of accusing someone asking a reasonable question of trolling.
Most people simply don't understand MySQL. MySQL is unique in as it support many database engines , it's like a common SQL layer on top of dozens of database engines. MySQL had a bad reputation because of its old MyISAM engine , the former default engine (one of many). The current default database engine is InnoDB  which is very fast and is what Facebook and many other huge web sites use.
Instead of repeating myself and others, MySQL+memcached is quite popular, check out the highscalability website .
 http://en.wikipedia.org/wiki/PostgreSQL , http://stackoverflow.com/questions/12622524/postgresql-9-2-1... , http://stackoverflow.com/questions/110927/would-you-recommen... , etc.
They were acquired by FB.
Also besides FBSON, most of what I've read in those slides looks like PostgreSQL's JSON support:
: JSON type http://www.postgresql.org/docs/9.4/static/datatype-json.html
: JSON functions http://www.postgresql.org/docs/9.4/static/functions-json.htm...
Does anyone know if Facebook is planning to open source it, or if not, whether something similar exists?
An alternative is the JSON Labs Release of MySQL 5.7.7 . The differences are explained in the slides, at the end.
Another great alternative is using PostgreSQL that supports JSON columns stored in a binary format since version 9.4 .
That said, I really like the approach of Facebook with DocStore, because of its use of "document paths", instead of functions.