Making contact...again.

So I have a peculiar situation, which I need some help in resolving. I've known I was adopted for as long as I can remember. My parents were always open about it The adoption itself was semi-open--my birth mother used to call my parents maybe a couple times a year, and I would speak to her on these occasions. She broke off contact for a period of time due to a marriage and divorce, but then reestablisheasd it later. The last time she called, however, was six or seven years--maybe longer (I can't remember all too well--I was young). The last time she called was when she came out to her family about the adoption. I talked to my birth grandmother and cousins at that time as well. She had also just gotten married again, and her and my adoptive mom exchanged email addresses, with the intention of keeping up regular contact. However, the email address didn't work for some reason, and she never emailed us. That was the last I heard from her. Of course, I didn't really understand what any of this meant at the time--I was just a kid, and though I had a vague idea of what a birth mother is, it wasn't something I actually understood. Now I'm an adult, however, and I have recently been searching for my birth mom. I found her facebook about a year ago, but I have been afraid to contact her. Whenever I think about contacting here, the one thing that comes to mind is "Why did she abruptly end contact?" There's always a little voice that says "Maybe she wants to talk to you, but just not now." And there's always a smaller, and more upsetting voice, that says "Maybe she doesn't want to talk to you. Maybe she's happily married and has moved on with her life, and you would just be an unwanted intrusion." I know, or hope I know, at any rate, that the second voice is a lie, but the first one I can't get over. I guess she could just not respond, but I have seen stories on here where even a letter or an email has upset someone's entire life. I don't need to worry about my adoptive parents approval, thankfully. My adoptive mom has always encouraged me to contact my birth mother some day. However, even if I were to contact her, I wouldn't know how to do it. Sending her a facebook message saying, "Hey, I'm your birth son! Call me!" might not be the most prudent choice I could make, but I'm not entirely sure what the prudent choice would be. In other words, how does one go about writing a letter/email/facebook message to one's birth mom indicating that one would like to reestablish contact? What would I even say? Every time I work up the courage to actually contact her, this is the part that I get stuck on. Thanks for any advice.