Immortal Fortress Twitter - http://www.twitter.com/immortal4tress[]
As above, this is the "official" Twitter account.
Adam Glover's Twitter - http://www.twitter.com/glovers_travels[]
This is the Twitter account of an Adam Glover, who appears to be having somewhat of an identity crisis. For more information, see The Story So Far.
Adam Glover's Yelp reviews - https://www.yelp.com/...[]
This was the first set of clues in the game, which provided information on the ten dead drops as well as at least one clue used later in the game.
Adam Glover's Instagram - https://www.instagram.com/glovers_travels/[]
The next step in the game... At the time of this writing, it is still actively posting.