If you have a pulse, you probably know what Nielsen Online just confirmed: Facebook is hotter than MySpace, growing its unique audience by 1.5 million people during October. But MySpace, which grew by about 300,000 people in the past month, remains the top social network, while LinkedIn had the biggest year-over-year gain with a 189 percent jump.

Viewed overall, the numbers suggest good news for the social networking space, with only a handful of sites reporting a drop in audience. Facebook, which had a 125 percent spike in audience, continued its summer surge into the fall, a trend that should please Microsoft, which last month invested $240 million in Facebook. 

According to Nielsen, the gap between MySpace, which had only a 19 percent increase in audience, and Facebook still remains large, with MySpace logging 58 million unique visitors, compared to 19 million for Facebook.

But as a CNNMoney blog pointed out, numbers for social networking sites are hardly foolproof. Both MySpace and Facebook have routinely reported numbers that differ from estimates provided by third-party firms. Facebook has long maintained that focuses on active users rather than total traffic.