Realistically, there's only a few people it could be. Out of the main five only Dan would be an option, because "Lonely Boy" who doesn't quite fit the description of 'Manhattan's elite' sure seems to get featured a lot on the site and is always conveniently away whenever the gang is texting GG, and when "she" came back to steal her laptop he was right outside in the car.
With the gang's most infamous foe Georgina eliminated in the centennial episode, this leaves our other two likely suspects - Lonely Boy's little sister J who could not only use Gossip Girl to manipulate her way up the UES hierarchy but also to keep tabs on her big bro; and Lonely's Boy's even lonelier ex Vanessa who never managed to take a liking to his friends and yet took to their lifestyle of plotting and scheming like a fish to water.
Another fairly obvious contender is recent returning character Nelly Yuki, former minion and accomplished writer (hint hint) but the character was originally created to replace Kati Farkas so all signs point to her being a red herring.
Finally, the show could shock us. The writers have struggled to surprise us for quite a few seasons and after what feels like fifteen Chuck's biological mother twists and about fifty convoluted reveals as to why Bart had to fake his own death involving horses, microfilm and Ivy Dickens seducing old geezers, just once the show could give us a simple yet genuinely shocking reveal. Dorota, if that is even her real name, has been at Miss Blair's side since the outset, privy to her deepest thoughts and indiscretions. For 6 seasons she's seemed remarkably composed, supportive and discrete despite Blair's many childish schemes and outbursts. Either Blair is paying her millions or Dorota and Vanya have a punching bag installed at home.
Or do you think it's someone else? Eric, who despite trying to kill himself decided he should run a website to torment his sister and her friends?Juliet, who was initially just out to get revenge on Serena, but simply got ridiculously carried away?Or another minor character that won't make any sense at all, but hey, this is Gossip Girl after all.