The user base of both Skype and Facebook is huge. They are big names, well known names. Known far outside the geeky underbelly of social media.
They are trusted names.
Combining the two adds to that trust. If Facebook and Skype trust each other enough to jump in to bed in a big way then that reaffirms and magnifies the trust placed in either (or even both) by the end user.
Most people are only going to want to video chat with one person or, at least, one location - (think grandparents on the other side of the world) and, as most video chat is likely to be between loved ones, they are probably already connected on Facebook.
It is always said that the reason Apple products fair so well (especially the iPhone and iPad) is because of the user experience.
If Facebook video chat is as easy as two clicks to connect then the experience shines through. Combine this with the fact that it looks nice, a clean video overlay, and there are going to be a lot of very happy Facebookers.
Hangouts on Google+ will attract a different - probably significantly smaller - demographic. Co-workers, media employees, school kids working on a joint project, maybe even gamers not having to resort to a paid service such as Teamspeak ( 10 World of Warcraft guild members, audio only, during a raid anyone?) Once the novelty wears off I can't see too many casual users diving in.
More than enough
Facebook may not have group video but they do have something: video chat is live now - not in field trial - and it is available to all. That's more than enough for anyone.