I can empathize with the OP - both my DD's (age 10 & 16) would practically move in at their best friends' houses, if I'd let them! It took some getting used to! But I limit their time at their friends' houses - absolutely no school nights (not feasible with swim practice and homework) and maybe the occasional weekend. The best friends can also sleepover once in a while. We include the friends in the occasional family activity too.
The stepchildren (especially stepson) were granted entirely too much freedom (not my idea, I might add) and it proved to be a disaster - long story that I won't get into here.
I learned to be a lot more cautious with sleepovers. When older DD was about 8, I allowed to her and my stepdaughter of the same age to sleep over at a friend's house with the understanding that I would pick them up at between 6:30 and 7:00 p.m. after I finished work. (It was during the school Christmas break - school night sleepovers are a no-no!)
During the late afternoon, DD called me three times with in the space of an hour with some odd requests. My alarm bells went off and I left work immediately and went to the friend's house.
When I got there, I discovered 4 very scared, defenceless little girls in the house alone and it is now dark outside. The host mom left the house since about 10 am and had not been back home. Except for a brief stint where she left one of her male employees to keep an eye on the kids, the girls were left unfed and unsupervised for HOURS! I called the mom of the other visiting friend to let her know what was going on. I was quite prepared to take the visiting friend home too, but HER mom was too busy with her "rendezvous" with her boyfriend! She didn't want me to bring her daughter home!
Much as I wanted to snatch up my two girls and go home, I could not in all good conscience, leave two 8 year olds in a house alone at night! So I stayed there for another 45 mins and then the host dad shows up, and he doesn't seem the least bit concerned about the fact that these 4 kids were unsupervised for almost the whole day! I don't think he's ever going to forget the tongue-lashing I gave him! The girls didn't call earlier because they knew I would have been very angry! I had to add "you must call if you are left alone at your friend's house" to the list of rules! It never occured to me to spell that out, but it never crossed my mind as a possibility!
The host mom called me TWO WEEKS LATER to explain. I can't remember the excuse, 'cause I just dismissed it - it wasn't a life-or-death emergency which to me would have been the only acceptable excuse! I still can't fathom why she didn't call. DH and I were all 5 - 10 minutes away, and would have been there promptly had we known what was going on.
Needless to say, our girls were NEVER allowed to go to that house again. Turns out there were some domestic violence issues there too, but thankfully my kids were not exposed to it!