According to Ironforge, almost half a million unique players were detected raiding in Blackfathom Deeps, looking at the timeframe from January 3rd – January 9th only.
These charts do not account for players that do not have raided within the last few days, or have their Logs uploaded to Warcraft Logs.
It does only require one player to upload Logs from Blackfathom Deeps, which has only 3 days lockout, but it is still highly assumable that many players are not included in these charts.
There is also a large crowd of players that have finished equipping their characters for Phase 1, play the Ashenvale PvP Event or explore the world which are also not included in these charts.