I have been getting reports, as has GigaOm, about a Yahoo Mail outage over the weekend for some users. According to Yahoo spokesperson Karen Mahon, "A small fraction of Yahoo! Mail users (less than .3% actually) experienced e-mail delivery issues over the weekend, but the problem has now been resolved. We believe this was an isolated incident."
If my math is correct, Yahoo Mail has about 250 million Yahoo Mail users around the world, and .3% would be 750,000 users--a small fraction on a grand scale.
Update: The incident began around 11:00 AM, Friday, February 9, and was resolved by 12:00 PM, Monday, February 12. Mahon further clarified that the software problem impacted .3% of the "system," but Yahoo doesn't have a breakdown of how many mail users were affected, which would depend on how many of them were trying to access their mail. So, the number of Yahoo Mail users impacted is somewhere between a few dozen and 750,000.
Regarding why Yahoo didn't proactively contact users about the issue, Mahon said that the problem was isolated and unexpected.
Given the potential number of people impacted and length of time of the problem, Yahoo should be more transparent is providing feedback to the affected users and about its performance problems.
For example, salesforce.com has a public Web site that provides real-time information the availability of its servers. If fact, every on service provider should have a public page that indicates the health of its services. Increasingly, reliability, performance and security will become the key factors that not just businesses but also consumers use to make usage and purchase decisions.