Well it depends on what you are targeting. If you are in these sites for fun, then obviously facebook and youtube are the best. If you are looking for some online marketing and exposure for your sites or blogs, then facebook, linkedin and youtube would be the best.
I experienced Twitter works far better than Facebook in helping your site to perform better on Search Engine
Facebook, facebook and facebook if you use it properly, you will need no other social network to grow
Facebook is better for me , here i created a fan page , here i can share some more fun things , and then get more fans so bring some traffics to my site .
Facebook is very effective in social connectivity while on other hand the twitter is used for tweeting that what you are doing currently or record a move of a twitting person. I guess to promote the website facebook is more effective where users can go though your status updates while on other hand if your tweet is not interesting no one is going to be notice about that... but in facebook your post or status update have more visibility... because it is seen by your fans... and that make it more convenient for popularize your posting of any website.. but for getting the quality traffic from facebook you should put the informative as well as attractive subject in status update.
Twitter gets more effective when you what to hit the target, And fr facebook it eats up all your time. But they are the movers of our time...simply make the best out of them.