C.S.
I'm not usually a fan of the "I'm the mother" routine, but it might be called for in this case LOL
You might set aside one weekend where sleepovers are allowed each month (or even two a month, if you're feeling generous about it). Plan some activities for the Friday nights when friends aren't allowed to join you. Cook supper together, play a game together, make time to sit and talk together.
You might also try to plan an evening for just you and one of the kids. Have your youngest help plan the weekly menu, make out the shopping list and help you at the grocery store, if money is too tight to allow for a "special" outting (which I SOOO understand). Your son could help prepare a meal (maybe a homemade pizza?!) to get him to feel useful and involved.
One thing my brother and I really loved, even into adulthood, was getting my dad to read to us. Several of my favorite books are "favorite" because of the act of sharing the story. It's like watching a movie, but it takes up several months of Friday nights! Not to mention it was good brain excersize and made certain we had his undivided attention (and he had ours).
Declare some evenings off-limits to friends and the phone. Say "we're going to veg around the house BY OURSELVES tomorrow night." Try to get the kids to pursue some artistic or athletic endeavors by themselves. A pile of paper, a heap or crayons and a pencil or two can keep my crew, from 1 to 10, busy for an hour. When we go for walks together, I can usually even get my oldest (who never stops talking) to be quiet and think for at least two minutes at a time LOL
Find something on tv that everyone can share. My kids have weirdly gotten "into" basketball this year. I personally despise the sport, but I love the time we spend plunked in front of the tube watching games as a family. We cheer and boo and argue, and even the baby gets involved in the clapping. My oldest, who won't sit through anything that doesn't have swords, lasers or animation has even taken to watching, just to be out with us. The best part is that I don't have to scan the plot for sexual or violent scenes that are not appropriate for any of the kids!
I'd say just be honest with the kids. "You need to learn to think your own thoughts, spend time with your family and I need some time without other kids. When I say 'no friends,' I mean it, and I'm the mother."
Your oldest is starting to unravel the apron strings, which is right and good. But he needs to also learn to build those family ties that keep us grounded and annoyed all through our adult lives :)
Good luck and take care!