Keynote said sites that usually load in two or three seconds were taking an average of 12 seconds or longer.

Problems with Web sites not only frustrate customers but also deprive sites of advertising revenue, especially if a visitor doesn't come back to view or send a card.

The Web site for Hallmark Cards Inc. was inaccessible or slow starting about 6 a.m. EST, according to Keynote. It was back to normal by late morning but got slow again around noon - just as people on the West Coast got to work and opened cards that had arrived in their inboxes, said Shawn White, Keynote's director of external operations.

Keynote said it also spotted problems with 123Greetings.com, and The Associated Press found in a test Thursday afternoon that many pages took several seconds to load. But officials at 123Greetings insisted that less than 1 percent of its visitors experienced slowdowns.

"123Greetings.com experienced record traffic today, one of the highest in its history," the company said in a statement. "Problems at other greeting cards sites caused record volumes of users to come over to our site, and we were happy to welcome them all."

• Approximately one-fourth of individual valentine cards are humorous, with adults aged 35-and-under being the most likely to send humorous cards.
• Valentine's Day is the largest e-card sending occasion of the year. An estimated 14 million e-valentines will be sent in 2008.
• Greeting cards are traditionally the most popular Valentine's Day gift in the U.S., ranking ahead of candy, flowers or dinner out.
• American men may be more serious about Valentine's Day than women. In a national survey for GCA in 2007, 45% of women said they were likely to give a humorous valentine to their sweetheart, compared to only 34% of men.
• The percentage of individual valentines exchanged through the mail in comparison to hand delivery is approximately 50-50.
• Red is the most popular color choice for valentine cards, follow by pink and then white. Hearts, roses, Cupid and lace are traditional valentine card icons.
• Teachers are said to receive more Valentine's Day cards than anyone else in the U.S., largely due to the tradition of classroom valentine exchanges.
• Men tend to purchase more expensive and more romantic valentines than women.
• The first valentines in America were exchanged during the Revolutionary War period. They were handmade and typically featured sentimental verses written in flowing script.
• Esther Howland, a young woman from Massachusetts, was the first regular U.S. publisher of Valentine's Day cards. She sold her first handmade valentine in 1849.