The 63e (a regiment) had the biggest and most successful NA server on the entire game for years. We recruited tons of people because of it, and when we would go there after events and form lines and actually use tactics, the pubs would follow along. We got plenty of recruits this way. When people see players in tags performing well and using teamwork it makes them want to join that group.
The 63e siege server was never the largest or most successful server for years. If you were to say it was one of the more successful servers, i would agree. It followed a pretty consistent pattern of being active during steam sales, to get all the recruits possible, then die off not too long after the sale ended. Admins would be rare and trolls would be constant after the sales ended. It was pretty obvious its main purpose was to be a giant recruiting platform. Which is fine. A lot/most regs used their server for that purpose. There was a consistent pattern that i seen for years where it was active when new players were in abundance, then died off until the next sale. It wasn't a consistently populated server. There were a few times it broke the trend, but it eventually died off.
On a side note, i hopped into the server a few times after a 63e event ended. After the event, they would send the regiment into the server. There would be 10, 20, or even 30 63e members just sitting in spec and not playing. Many of them asking if they could leave yet. The 63e used their large numbers to inflate the servers pop, as people like to join servers that are populated to get it going, then most of the reg would leave. It was pretty effective. It is a great way to get an empty or near empty server going. It was a tool that most regs or groups wish they had.
BBGs bot battle server, and even NA1 are or were the most consistently populated NA servers over the years.(BBGs bot server started in 2015 i think) BBG is still going strong with several different servers. NA1 died off 2 or so years ago.
Im not going to speak for the EU servers, as i generally didn't play on them all that often.