That's a lot of virtual messages delivered in only a couple of months. Actually, most of that traffic has been during the last two weeks. Oceangram.com has been BUSY. If you check their Clustr Map, though it hasn't gone live yet, you can read that visits on the previous 'day' were 62,785.
My prediction earlier that the 500,000th bottle would be delivered at around 5:00 am GMT was pretty close. I expect it was at about 5:31 GMT.
If you saw 500,000 on your beach and captured the image, please comment here.