This news means that for the March-May period, the daily combined Bitcoin transaction volumes in Overstock were around $8,000. This comprises less than 1% of the Overstock sales.
First of all, overstock is not all that well known outside of America (at least I personally never heard of it before bitcoin). Even though they do deliver here as well, apperantly.
Second, the percentage of people in the world that use bitcoin is far less than 1% and the total amount of value in bitcoins is not even remotely close to 1% of the total value of all currency in the world. And I'm not even talking about the combined value of gold, silver, platinum, stocks, bonds, oil, etc.
Bitcoin may look big with it's several billion 'market cap' or whatever you call it in bitcoins case, but in reality we are not even a tiny drop in the bucket yet. Which is not at all bad because that only means we have plenty of room left for (exponential) growth.
Considering bitcoin is so small it's a pretty big deal even if overstock would not really miss a couple of thousand dollar a day.