Update: Hulu is reportedly critical of the latest ranking it has received from Nielsen Online.

Nielsen reported 8.9 million visitors to Hulu in March, while comScore said there were 42 million. There were 373 million videos viewed on Hulu in April, up from348 million in March. Even though the number of videos viewed was up, Nielsen said unique visitors to Hulu had declined from 9.5 million in February to 8.9 million in March to 7.4 million in April.

Jean-Paul Colaco, the senior vice president for advertising at Hulu told the New York Times "there is more work that can be done with all of our research providers to ensure that online video metrics are accurately represented to users and clients."

YouTube continued to be the top video property in April with 5.5 billion total streams, according to Nielsen online.

Hulu ranked as the second most popular video site during the month, increasing 490 percent in total streams year-over-year, from 63.2 million in April 2008 to 373.3 million in April 2009, making it the fastest growing brand among the top 10.

“Historically short form, clip-length video has ruled streaming on the Web—as demonstrated by YouTube’s top spot month after month,” said Jon Gibs, vice president, media & analytics, Nielsen Online.

“Hulu, along with pure-play providers like Veoh and the TV networks, have spent the past two years trying to convince consumers that the Internet can be a good place to watch full length programming as well. April’s strong showings of Hulu, Fox, and ABC suggest that consumers are beginning to listen.”

During the month, people between the ages of 35 and 49, were the fastest growing demographic in time spent viewing, increasing 29 percent during the past six months. This was 13 percentage points higher than the growth of time spent viewing for the overall market, which increased 16 percent over the same 6-month period.

“Despite what many believe, it is not the young, tech-savvy, early-adopters who are attracted to long-form video. In fact, we see that it is the older crowd, viewers 35+, who gravitate toward long-form video, with sites like Hulu acting as a perfect example of this,” said Gibs.

“And advertisers are paying attention. They see long-form video sites like Hulu as a safe bet for online advertising, recognizing that their key audiences are there and more willing to sit through an online advertisement while watching a favorite show, much as they do with the TV.”

Since its launch, Hulu, the joint venture between NBC and News Corp, has experienced booming growth in all aspects of view viewing, especially time spent. Total time spent viewing increased 119 percent between November 2008 and April 2009. As more people visit Hulu and stream more videos, more time is also spent watching these videos.  Time spent viewing increased 120 percent from 147 minutes in November 2008 to 325 minutes in April 2009.

Driving Hulu’s rapid growth are unique viewers between 35 and 49, who in April represented 30 percent of total Hulu viewers. They also spent more time on the site than any other age group, with an average of 416 minutes spent viewing per visitor during the month, 10 percent more than any other demographic on Hulu. During the past six months, time spent per viewer for the 35 to 49 demographic group has increased 154 percent.

“Hulu’s growth in time spent viewing illustrates that they are setting the pace in the video market. Although this growth has a lot to do with a good interface and excellent programming, Hulu’s aggressive marketing campaign, starting with their first TV ad at the Superbowl, seems to have given them a bounce that they haven’t yet fallen from,” noted Gibs.