X-Fire rankings are interesting, but I am not sure what it means - if anything. X-Fire is typically used only by the hard core gaming community as a measurement for activity within a specific game. It doesn't reflect trends of anything other than X-Fire users, which have proven time and time again to not accurately reflect a good sample of gamers in ANY game.
So the hard core gaming guilds are moving on? Shocking, they do that – a lot. They are the flavor of the month types and few have any longevity in any game - and NEVER have. Those guilds grow and claim fame by recruiting new people thus increasing their variety of experience in games, not actual loyalty to any specific games.
There is only one thing I think the X-Fire ranking says about STO - if there was momentum at launch it has since faded. That actually means little in MMOs. I don't think the longevity of any game can be determined in the first 6 months, much less the first two.
With the way XFire is working currently with STO, it may still be registering people playing that haven't logged in for a week or so. The statistics are meaningless since I have been playing 24/7 for just over a week.
It's typically not the best idea to trust a third-party, opt-in source to divine user numbers. I believe there's some technically wonky stuff between the way Xfire and STO interface with each other, as well.