I still visit the place to moderate and such but there isn't a lot going on, today is the first report I've had to deal with in a month when I used to have to deal with maybe 6 or more a week.
As soon as the gang system died out I lost interest pretty quickly. Get the gang system going again and you'll get activity in the private forums. Get that started and you'll have the large amount of new members noticing this and wanting to join a gang. If this happens, they'll continue to return as they feel they are a part of the community. I know the reason for removing the private topics was to help encourage those members to post in other parts of the forum, but ultimately it stops member activity as they don't feel as inclined to return to the forum.
Competitions might increase activity for a wee while, but after their over you'll notice activity dies fast. The same applies to business; if a company offers lets say a 2 for 1 deal, it will increase sales while the offer is ongoing but because it's doing nothing to build upon brand loyalty for the company, as soon as the competition is over sales will fall right back to where they were before.
While a revitalisation in certain parts of the forum would help, there really isn't much we can do to increase activity for the long term until news for the next GTA starts to come out. Best thing I think to do is to revive the gangs. Bring back the private forums. Lower the requirements needed by each gang for members to be accepted into a gang. But most of all, the older members who have been here for a while need to make these new members feel a part of the community.