There is a wide range of social network connections policies at an individual level. This is, how I consider who I invite and who I accept on social networks might be different from what you. But most people that I know use two different standards: One for Facebook and one for LinkedIn.
Facebook has the widest range of "personal policies". I know people that will accept any Friend invite, no matter if they even heard about that person before. I also know a person who is using Facebook as an exclusive "family network". Seriously.
On LinkedIn I see mostly two standards: 1) Those that accept/invite anyone they've done business with, or would like to do business with, and 2) Those that are only people they've met in real world.
My social network policies are these:
- On Facebook I'll invite or connect with anyone that I know (physically or virtually) and that I know he or she knows about me.
- On LinkedIn I only invite/accept people I have met on real life.
- On Twitter I'll follow mostly people that I've met, know personally and find their tweets (mostly) relevant to me.
The sucky part is that I have not found a way to get my LinkedIn connections into FB, or FB into LinkedIn, or to find out about my existing FB/LinkedIn friends that just joined Twitter. I'm sure there is 100 early-adopter tools out there, but I'll wait for the "de facto" to rise to the top.
I'm the Co-founder & CTO of 
