Re-read JB's answer. That's our situation too and always has been.
Did you, yourself, grow up in a neighborhood where kids just tumbled out the door and disappeared to play with other kids all day and the notion of actually scheduling get-togethers was just never thought of? Sometimes if parents grew up like that themselves they forget that things are different now.
Your sons are all old enough that they and you can and should be doing meet-ups. If they're in activities of any kind, you need to get to know the parents of the kids they like at those activities (I'm not saying become best buddies, but introduce yourself enough that the parents are fine later when you get in touch about the kids meeting up). Same with school -- it really helps to be acquainted with parents of kids your boys like. Ditto for church if you go to one; think about what kids there your kids are pals with and see them outside the church setting.Talk to your sons and just ask them what kids they'd like to have over to play, or to meet at a park, etc. If you don't ask, they may not think to tell you, because it just does not occur to kids that they can ask to do things, sometimes.
Once you have their ideas, you can be proactive about e-mailing and calling to set things up. Your older boy can talk with his friends and then talk to you about "I want to ask Billy over on Friday after school to play in the yard and maybe play computer games afterward." Eleven is still too young to set things up entirely without a parent involved; at 11, kids don't know their own schedules all the time and can't drive themselves, so you do have to play a role here and will for years to come.
This topic comes up on this board pretty frequently, but frankly it is just not essential to kids' lives to have neighbor kids they can run off and see any time. That's not how it is any more in many places -- yes, in some, but in many areas kids, especially as they get older, have activities (sports, Scouts, church groups, school clubs, dance, drama, whatever), and those activities mean they are not available to hang out in the neighborhood. BUT those activities also give kids a great potential group of other kids with whom they share real interests -- and shared interests are the basis of real, deeper friendships. Mere proximity, just living on the same block, does not necessarily make friendships for life.
Please try not to feel down about it. Take that energy and turn it toward getting out there and taking the role you have to take if you want them to play with other kids. Give up mourning the neighborhood and have your kids seek out friends in other places. You will have to do more driving and e-mailing and phoning but it's worth it.