They are really quite fun! I did one for a book release on my own and I have done "take overs" with other authors too. The basic premise of having a party online is to get the audience excited about your book. These types of parties work well for people that have a following outside their hometown because traveling might be hard for your readers that are across the country or across the globe.

A "take over" is effective for newbies because you are exposing yourself to other authors AND readers. Most "take overs" happen in one hour time slots and the party itself can last one day to a month depending upon how big it is. You can find these while scrolling through your invites on FB because most of the time FB will suggest a party for you to attend.

A few tips for those that would like to try this:

1. I do not suggest that you pay anyone to join one of their "take over" parties. Some businesses will charge you because they will post on your behalf, but from what I have personally seen first hand-- this can backfire on you. The few authors I had seen do this on an event did not get that many people adding comments. Of course, there's always the flip side where it can work. Just keep the goal in mind that you want to expand your readership with takeovers. 
2. If you have a themed book-- go for a themed party. For instance, I write paranormal and I will look for "take overs" in and around Halloween. That's the best time!
3. If you decide to throw your own shin dig, make sure that you send out status updates about the party. Don't just invite people through FB and Goodreads. Some people do not typically look at their calendar on FB because they are not alerted in their updates anymore. 
4. It also doesn't hurt to ask other authors if they'd like to join your shin dig. Many authors will jump at the chance!
5. Plan out some great content for your party ahead of time. You can have your readers go on a "scavenger hunt" that leads them to your blog, website, newsletter, Twitter, or whatever you desire but get creative with the hunt. I had sent my readers to several social sites and my website by posting questions about the book. They had to go to said web/social site to read teasers on those pages for the answers. You can have themed music playing in a post by adding a Youtube video-- perhaps one of the songs that inspired you to write the book. And most important! Be sure to post your trailer if you have one and LOTS of pictures so the party has visual fun too. Many romance authors post Memes and "Eye candy" but I've done pictures of vamps, werewolves, etc. The sky is the limit with this creativity. Plan to host at least 4 giveaways (about one every 15 minutes) during your hour long event so you can keep things moving. If you are running your party for longer you can do more giveaways, but keep your budget in mind. Swag, books, bookmarks, and postal service can get pricey-- especially if you ship overseas. Some authors do two separate giveaways in this particular situation and offer an eBook to anyone outside the author's country. 
6. Lastly-- DO NOT be shy! Chat it up throughout your party. You may feel like your are talking to yourself at times-- but remember not everyone will show up "LIVE". Some may stop by an hour later or even the next day because of time zone issues. So make sure your contests are open for at least a day to give the stylishly late party goers a shot. Can't figure out how to pick a winner from all the cool comments? No problem! Go to Random.org! The site will randomly choose a number for you. You can also try Rafflecopter, but I suggest Random for FB events. It gets people to comment on your posts and have some fun.