You asked

What are the best social networking sites for tweens?

Here is a list of some of the safest and most popular social networking sites for tweens:
Stardoll : Combines social networking with online gaming. Girls ages 10-17 can play games, create a MeDoll and chat with cyber friends if given written parental permission. Stardoll also notifies parents if kids are on the site for an extended period of time.
Whyville: Perfect for budding entrepreneurs and politicians. "Citizens" can write for its newspaper, start a business, buy a car and earn "clams." You cannot access the site unless you receive parental permission and take a "chat license test” first.
Moshimonsters: On this site, kids take care of creatures to earn points to buy virtual items. Games are more entertaining than educational and it's best for kids 9 and up. The site makes sure to collect parent information and verification for kids under 13 who register on the site.
Yoursphere: This site lets kids set up a page, friend other members and join "spheres" about certain TV, movies or music. It features a Facebook-like news feed and kids can even blog, play games and participate in chat rooms. Yoursphere — best for 9+ — vets sign-ups against a database of registered sex offenders and asks parents to register and undergo identification verification.
Fantage: Fantage is a free, virtual world site for tweens where they can ride rocket boards around animated castles and cities. Kids go on quests and request buddies, block users and IM — which is filtered for profanity and personal info. It's best for kids 9 and older.
Franktown rocks: Franktown Rocks is the best site for kids interested in music. Kids trade in "Franks," earned by playing games, and use the Franks to buy items in the virtual world. Content is monitored and chatting can only be conducted through pre-selected words. The site is best for kids 9 and up, and helps foster music appreciation, with music choices that even tweens will find 

More questions

As smartphone use increases in popularity, so too does the risks associated with them. These types of phones create easy access to social media sites but unfortunately, a lot of young children are not aware of...
If you have a tween at home, you will be aware that tweens are very tuned into the newest, coolest status symbols around...
Establishing rules about technology in the home can be difficult. However, like games consoles, television and the internet shouldn’t affect family time together.
Trying to decorate your tween’s bedroom can be tricky to say the least!
Here is a list of some of the safest and most popular social networking sites for tweens:
Sitting at a computer or console for hours on end, seven days a week, whether it's for fun or for work, isn't healthy for anyone, especially not for children.
The internet can be a wonderful educational tool for your child's learning. 
Getting your child to study and not spend their time on facebook can be difficult but there are lots of things parents can do to promote study.