Let's Encrypt's root is not trusted by any browser yet, so they got a cross-signature from IdenTrust. So far, basically everybody using Let's Encrypt chains to the IdenTrust root.
Given that IdenTrust grew from "<0.1%" to 5.6% since the launch of Let's Encrypt, and the shape is roughly the same as https://letsencrypt.org/stats/ , I strongly suspect that the 5.6% market share credited to IdenTrust actually belongs to Let's Encrypt.
As a check, we can divide 2/3 of the total unexpired Let's Encrypt certificates (~2.8M as of last week, 2/3 because standard practise is to renew a 90 day cert after 60 days, so 1/3 are probably overlapping = 1.86M) by the total number of https sites (I had a hard time finding this, but http://arstechnica.com/security/2016/03/more-than-13-million... claims that 5.9M is 17% of https servers, so 34.7M), and we get 5.4%, which is almost the same number as the IdenTrust number above.
That's still nowhere near Comodo, but it's not completely insignificant.
This is correct. (Source: I run Let's Encrypt)
Comodo and Synantec are 32% each, then goes others. StartSSl is significant but 2%, Let'sEncrypt tiny for now: https://w3techs.com/technologies/history_overview/ssl_certif...