As reported on the Second Life blog, during 5:20pm and 5:45pm PST the concurrency reporting services where updated which caused a drop in the concurrency numbers, but not to the actual online residents.
Edit: This was a Tuesday concurrency record as well.
Statistics compared to Tuesday, February 12th:
Peak concurrency grew 2,454 users, a increase of 4.31%.
Minimum concurrency grew 2,261 users, a increase of 7.40%.
Median concurrency dropped 269 users, a decrease of 0.59%.
There where several issues last week which affected the concurrency services, a big chunk of stats during the daily low weren't retrievable. This skewed last weeks median upwards, this explains that Yesterday the minimum and peak concurrency both were considerable higher and the median slightly lower then last week.