Dirt 3 was released 7 years ago. Since then, 3 other Dirt games were released. As decent as Dirt 3 was, yeah, most of its playerbase disappeared. It's still playable online, although the pool of available and currently played races is reduced due to the low number of players. Don't expect it to be as popular as FiveM which, not only was released on a recent rendition of GTA V (2015, compared to 2011 for Dirt 3), but was also released based on a GTA game, a series way more popular than the Dirt franchise.