Being part of the marketing team, and one of the social media accounts manager, I can tell this is just a matter of manpower. We have distributed the social media accounts between us, and it just happens that some people manage for some reason or the other to have more time to post on some of the social media services than others.
There's not strategy decision about post more on facebook and twitter than on the others, it just happen that we all give our best, but some have more time for that than others.
Having said this, if you want to help with this please contact me. We will try to integrate you within the team and make our presence on the privacy respecting social media networks better, by having the manpower to do it. We always try to have several persons for each social media network, but we can only do so much.