Anthropologist Robin Dunbar suggested that 150 is the maximum number of people that one person can have meaningful social relationships with. Check out Rebecca Goldstein for more on Dunbar's Rule. It's not a hard and fast rule. I know some people who couldn't have meaningful relationships with more than 5 others.
It's hard to fight human evolution and how we're wired. But we do change. Sometimes quickly, sometimes slowly.
We're starting to see some people experiment and push the upper bounds of Dunbar's Rule. Social media and social networking seems like a natural catalyst for this.
For those of us who want to expand the number beyond 150, I suggest applying another rule. Pareto's Principle or the 80/20 Rule. Concentrate on helping your network. Concentrate on only the 20% of everyone's stream that is important enough to have an impact on your life and their lives. Concentrate on meaningful.
"Listening" to your social network has become an exhausted cliché. It's how you listen and what you listen for that will help you break new ground and enlarge your circle.
This takes incredible discipline.