Tumblr post backlinks/root domain

Hi,

I just start using tumblr as backlinks (expired accounts)
I saw Google indexes most of the time the root domain and not the post itself.

The question is, should I build backlinks for the post where my money site link is or should I build backlinks on the root domain targeting my keyword?

I use >5 posts on the tumblr blog.

do you post on those tumblrs manually or u use any bot?

Manually