My son is 16 and he has always had just a few close friends. He is not a social butterfly and gets along well in the company of adults. While he occasionally has friends over to the house, most of his social interaction takes place at school, band events, or through the PS3 network.
I used to worry about his seeming lack of friends. And try to arrange plans for him, or encourage him to make plans with others. But, he finally told me to back off. He is happy and does not feel like his social life is lacking.
Birthday greetings on FB are not a good indicator of social popularity. My son has over a 100 "friends" on FB, but got only a hand full of birthday greetings.
Talk to you son. Since he is active in sports and youth group, he may feel like he gets enough socialization time with his peers through those venues. The key is to communicate with him and ensure that he feels okay with himself. Because that is all that matters.
Once he gets into college he will meet people. Especially if he lives in a dorm. LOL He sill meet peers through classes, campus activities, etc....like we did in college.
As for girls, mine didn't start dating until this year in school - and he actually only sees his girlfriend at school right now since her parents will not let her actually date.
Remember that your son is unique and all teens mature at different rates. I think sometimes that we Moms have more teen angst than our teens do.
Hugs.