Weak Ties are people you've met and with whom you maintain loose relationships.
For example most of my Twitter follower are weak ties (want to experiment? follow me on Twitter).
But Weak doesn't mean unvaluable.
For example, if you’re looking for a job and ask your friends, they won’t be much help; they’re too similar to you, and thus probably won’t have any leads that you don’t already have yourself. Remote acquaintances will be much more useful, because they’re farther afield, yet still socially intimate enough to want to help you out.
This is short quote from this article about the growing importance of feeds and I highly encourage you to read it if you want to understand how people get hooked to Facebook or Twitter.
