I ran into that as well. Despite having used the platform since 2007, I was unable to follow more than 5000 users (also despite having ~1400 followers). Considering the platform has some hundreds of millions of monthly active users, it was completely mind-boggling to me that I can't follow more than ~5000. Especially when you consider how many official orgs like local police, fire, emergency, road/traffic, news, school district, companies you are interested in, bands you like, etc... it's basically inevitable to hit that limit, especially after having used the service for 15+ years.