Now, the competitor that makes these charts very skewed and a variable that cannot be measured all that well is Furbooru.
This place stands as one of the most interesting options out of the bunch but it strays away from being a desirable non-intrusive option (and even in that regard, I raise questions to the amount of degree that they do so in practice).
Everyone would know at first,just by checking its staff members, that this was released as a project that was born out of Derpi and therefore, one must place it within its league. Nonetheless, there are factors to consider beforehand:

1. The amount of censorship and the overlap with its peers >>8197
>It is more than Derpi from my understanding of them. If you rate it by alignment that might a little more complicated but I would still lean to putting it next to Manebooru. I remember seeing SJWs smirking off to there over Derpi early on, but the owner doesn't want to deal with politics at all and (at least in the past) shutdown the attempts to start any drama.
It´s really hard to illustrate a general line of thinking around there. If one decides to check out the amount of overlap with other alternatives, you can see that a certain sector can either switch with Mane, Twi and Ponybooru (6,8; 6,5 and 5,0 respectively, making up a total of an 18,3%) and this is where it breaks the schemes of the whole thing. The altboorus that came out of /mlp/ were a reaction towards the excessive censorship whereas Manebooru did the same but towards the opposite direction. However, Furbooru had been created before Derpigate. One would suspect that they wanted it to mix Derpi and that site in order to become one. However, that idea could have been ditched at this point, leaving it as its own undefined project that embody the Philomena competitor against Furaffinity as well.

While I have ranked its censorship around the same as the one that would come out of Derpi, both the overlap with the different sites and how the staff actually apply their rules either follow the desires of those users, follow the same systemic moderation that they would apply on Derpi indistinctly or simply prefer to keep it politically neutral overall....it makes the whole judgement difficult.

2. The amount of users that want to use it as a MLP related site.
>we do not know the exact amount of overlap between the sites and the fandom at large. Furbooru could easily have a higher number who are from Derpi, considering their advertising has been significant there. Or they could be showing up in google images a lot durring Furry searches and the core fandom crowd might be a tiny minority of people at this point.
Albeit Alexa provides the numbers for Mane,Twi and Ponybooru in the overlap department, it doesn´t display the ones that come from either Ponerpics nor Derpi (the most important one). Considering that there are around 155000 images and 25100 of them are related to MLP, that makes up around a 16% of MLP content within the site. However, there could be fans that use that site because it also provides furry content with a proper order (unlike Deviantart) without having to move onto another place. Therefore, I have estimated that a 20% of its userbase searches over there for MLP content or at least, said users could easily use other MLP related sites.

Here lies a huge problem for quantifying the exact numbers of those who might be interested in MLP to some degree. This hybrid site represents a collaboration of both communities (probably a bold move on its own,considering their track record in the early 10s). As a result, these numbers are skewed by default and I would have to rely on an estimated percentage that is based on assumptions or indirect factors that could lead to a likely number to latch onto.

Out of all the boorus, as far as I am concerned, Furbooru raises a lot of uncertainty for both this chart and the project itself in general, hence it becomes very complicated to judge because of its outsider nature.