Let someone you invite to your network know how you came to find them or be referred to them. It can be as simple as, “I’m long time friends with so and so and I found your contact through her and think our connection here may be mutually beneficial.” Anything less than that is just creepy like the stock messages I’ve been getting on LinkedIn, “Please join my network.” My knee-jerk thinking is, “What do you think this is? Twitter or something? I don’t know you! I only talk to complete strangers on that social network.