It's just Pokemaster. He runs all the sites' social media on his own (mostly Twitter, according to him).
Website that revolve around images don't make much sense to invest time into. PM can just update the news page with images or tweet them out if they're central news. Imgur and Instagram are unlikely to bring in a lot of attention because they're used mainly for memes and other amusing content. Not really to follow sites like this one.
I obviously don't know PM's finances or any idea of how the traffic is coming along lately, but from what I've seen Twitter seems to just be PM's way of quickly informing followers about incoming information about Pokemon or the site. It's quicker and easier than updating the site everytime you wanna write a quick sentence, and it gets the information so a not insignificant amount of people quickly. Also from what I've seen people tend to go to social media from this site rather than the other way around.
If anyone knows anything I don't (coughpokemastercough) then feel free to answer too of course.