Who is the main target audience of yuri titles? I know that yaoi is famous for its fangirls, but I was wondering what the case was for yuri.

I have only occasionally seen yuri fangirls being referenced in anime, and these shows have a cast that is fully female.



Inspiration!

Researching it, there seems to be many subcategorys of yuri, which are targetted towards men, women and younger girls. But I was wondering - which is the main target audience for this genre?
Yuri is a genre.  It, like any other genre - drama, action, romance, adventure, slice-of-life, etc - is part of what will define the particular plot the show will have.

It by itself isn't a demographic, which is what defines the target audience.

Many yuri-centric works actually do have a demographic attached to it explicitly.  For example, Girl Friends is listed as seinen,  and Collectors is listed as Josei.  They may also have a demographic attached by virtue of the manga that they're published in, such as Kannazuki no Miko being a shounen manga, due to it being published in a shounen magazine.

In general, the demographic will vary, but I've noticed it by and large be seinen.