ZCABs right, we have tried a lot of things such as irc etc. But it was all abandoned. Whats hurting the activity most is the lack of information about GTA V and its almost 5 years to the day since GTA IV. When there is news, even if its not much, its a motive for users to post here and it does actually get busy but after 3 months of silent treatment it becomes inactive again.
Once GTA V is released, like every GTA release, the forums will be busy again. Its always good to see people give feedback and try to help bring some life