Facebook:
Facebook is not the first social network but it is the most popular one. There have been many other social networks before Facebook and while some of them were popular at some point in time, none could reach the popularity of Facebook. In addition to keeping in touch with your friends, Facebook can be (and is) used for business. You can use it to promote your products and services, to acquire new clients, or to get traffic to your site.
Benefits:
Your profile is your major weapon:
As with Twitter and any other social network, if you don't make your profile interesting, you will hardly become popular. Give enough background information for you and don't forget to make your profile public because this way even people, who don't know you, when they encounter your profile, they might become interested in you and become a supporter of yours.
Build your network
Post regularly
Be active
Arrange your page
Check what Facebook apps are available
Use Facebook Social Ads
Start a group
Write your own Facebook extensions
Use separate profiles
Twitter:
Twitter is simple to use and this is what made it so popular. Twitter is fashionable right now, so enjoy the moment. Even the creators of Twitter admit that as with MySpace and other Web 2.0 sensations, Twitter will inevitably go out of fashion some day, so hurry up and get some traffic for free now, when it is still all the rage of the season.
Benefits:
Make your Twitter profile interesting
Pick a niche-targeted username
Put your site/blog URL in your profile
Search for Twitter users with similar interests
Socialize on Twitter as much as you can
Tweet regularly
Don't spam
Take advantage of Twitterfeed
Make Twitter Search love you
Add Twitter gadgets to your site