I generally only add people in Linkedin that I have a personal or professional interest in. I try and make sure that I actually meet, in person, the most important contacts on my Linkedin. I try and add people who are high up in the companies I want to work for WHERE they are also connected to people that I personally know.

You need to get out there and grow your network in person as well as online. Online should supplement what you do in real life.

I also only add recruiters that work in the specific technologies that I use, I also try and call these recruiters for a casual chat about the state of the market. Only recruiters that I get a good vibe from remain on my list.

This has resulted in me never being out of contract work for a few years now and my network now includes CEO's and MD's at very good companies who I have personally developed relationships with.

Cultivate connections with people you know and trust, rely on their connections instead of trying to force connections with people you don't know.

The power of linked in isn't building a giant list of everyone who is related to you, it's building a tiny list of really awesome people who have a tiny list of really awesome people who have a tiny list of really awesome people...

TL;DR: If you curate content, don't be surprised if the quality rises.

EDIT: This is the perspective of a more introverted type who tries to build fewer strong connections instead of many weak connections. If you can build and maintain many weak connections effectively, go ahead! I just know that I absolutely cannot maintain those well, and prefer to network with those who can and rely on them to maintain those connections.