Great summary from Nielsen:

In the Nielsen Online March Automotive Industry Overview, we looked at key drivers of discussion within the industry and took an in-depth look at trends in unique visitors and automotive ad spend as they related to Facebook and MySpace.

During the past 13 months, a large shift has taken place between two social networking giants, Facebook and MySpace. Most significantly, Facebook recently overtook MySpace in monthly unique visitors and average time spent per person. Time spent on Facebook increased 187 percent year-over-year, from one hour and eight minutes in March 2008 to three hours and 16 minutes in March 2009.

The question remains, how does this shift in visitors and engagement impact advertising on these two sites? While Facebook’s ad spend has remained steady, MySpace continues to attract large-scale campaigns.

Among automotive manufacturers, Chevrolet, Ford, Nissan, Honda and Toyota, Honda led in online ad spend on MySpace and Facebook combined during Q1 ‘09. This lead can be attributed to a Honda Fit campaign that generated more than 600 million ad impressions on MySpace. Meanwhile, Toyota allocated more dollars on Facebook in Q1 ‘09 – the same timeframe in which Facebook overtook MySpace in unique visitors and time spent.

It will be interesting to continue tracking Facebook’s stats to see if the site builds upon its steady growth or if it begins to plateau. Perhaps more interesting though, will be if automakers begin spending more dollars on the more visited and stickier Facebook, especially as effective ad spend becomes increasingly important.