You're quite wrong here. Google, AT&T and Verizon on your list use OpenLDAP. OpenLDAP scales to billions of users in a single instance, and of course it distributes across multiple nodes for even greater scaling as needed. Most of the telcos around the world use it now; it scales far beyond every other data storage technology out there.

http://symas.com/docs/2014-LOADays-jkozyra.pdf

80% of the US population that owns cellphones are OpenLDAP users, whether they realize it or not. 90% of the US population that subscribes to cable TV (Comcast, Time-Warner, etc.) as well. Nothing else actually works for hundreds of millions of users. These companies know, they've tried a lot of "solutions" already.

Then I'm glad to stand corrected - I suppose I'm horribly out of date from the days they tried other "solutions". I'm less surprised about AT&T and Verizon, more surprised about Google.

My own personal experience with LDAP in general (and some with OpenLDAP in particular) dates back to 2000 or so -- and at some point I just gave up on the complex and unreliable beast that it was (for whatever reason, BDB would occasionally lock up or get corrupted) - but I know a lot has changed since, LMDB is amazing -- I guess it is time for another look.

OpenLDAP didn't start to become seriously usable until about 2003 (that's when HP dropped SunDS and adopted OpenLDAP for all of their global infrastructure). A lot of work went into it in the 2001-2002 timeframe to get it there. Yeah, a lot has changed, you've missed a couple generations of advancements. BDB's complexity was definitely a pain, but that's all in the past.