I have a few thoughts to share, after reading your post, the follow-up and everyone's messages...
1. If you want to get to know his friends better, have a BBQ and invite them all over so you can get to know them. Tell your son you're doing this so you can increase your comfort level with the people he's chosen to hang out with. If they don't show up...well, I think that says something about their character and their level of friendship with your son.
2. In my husband's family, curfews were always set by the level of responsibility each child displayed (my husband was one of 5 kids). My husband was curfew free at 17 because he showed great responsibility. One of my sisters-in-law had a curfew until the day she moved out at 20 because she showed poor responsibility.
3. When I came back home after my first year in college for summer vacation, I was 18 and automatically placed myself under my mom's high school rules. (Old habits die hard, I guess.) My mom changed the rules and said I didn't have a curfew but I had to let her know where I was going, who I'd be with and what time I'd be home (and I was allowed to not come home 2 nights, not consecutive, a week). Considering I had lived on my own for a year, had been in a steady relationship for over a year, was getting good grades in college and had a job, this seemed more than reasonable to me.
Having said all that, and rereading your follow-up, I would say that your son has a few milestones to pass before he can have a "no curfew" life. First, he needs to get a job. Somewhat looking doesn't cut it. Second, he needs to earn his driver's license and drive responsibly for at least 6 months (no tickets, no accidents). If you have to haul him around to do stuff, you get to call the shots. If his friends are hauling him around, they need to haul him home on time. (And, really, they didn't want to bring him home? What kind of friends don't take their friends home? Press pause on the movie or game, for goodness sakes. I'm glad you didn't buy that one.) Third, he needs to earn good grades in college. Once your son has shown this level of responsibility, then he can be released from his curfew. Right now, from what you've said, he has not shown enough adult responsibility to earn this privilege. Just because he's 18 and LEGALLY an adult, doesn't mean his thoughts, actions and day-to-day behaviors are those of an adult.
If your son doesn't like the rules you set, he's more than welcome to move out and live on his own...without financial assistance from you. If he can't afford to do that, or doesn't want the responsibility of providing for himself, then he gets to deal with your house rules. If your son doesn't understand this, then he may not be as grown up as he thinks he is.