I tried the new platform for the first time a few days ago, and saw that my rating there was around 4500. My rating on the last day of the Goko platform was about 5100, and through this year it's been somewhere between 5000 and 6000, usually around 5400 I guess. So I can only conclude that they took my rating at some point in the past, and since I haven't been playing on the new platform at all, it's been steadily dropping.
This seems to me mostly like a bug. I shouldn't be forced to play on the new and unfinished platform as long as Makingfun themselves deemed it not good enough to take down Goko. What they should have done is either: (1) Let everyone start at zero; or (2) export the ratings (at whatever point) and freeze each user's rating until that user actually plays a game on the platform, or at least until that user actually logs in. In the meantime don't even include that rating in the leaderboard.
Or maybe there's another reason for my current rating, some bug not related to a steady decline over x amount of weeks or months.