We have a variety of relationships with our daughters' donor siblings. (they have the same donor, phew!) We're not incredibly close to anyone of them, but we see one particular family several times a year. The little girl is 9 months older than our oldest, so they play well together. There's another family who live about 25 minutes from us and they have a little boy who is a year and a half younger than our oldest daughter and about a year older than our youngest. We saw them the other day and did a lot of comparing about their personalities (it's all the donor's fault! lol!) There are two other families who live several hours away, and we've met one of them, but are eager to get together as the kids get older. Finally, we met one family randomly on the beach (yes, it's true) and while I've run into one of the moms several times, she's always friendly, but seems completely uninterested in furthering the relationship. I think we've struck the ideal balance for us. We can hang out, they send birth announcements, and we exchange pictures, but it's not like we're trying to raise our kids as siblings. I know some people want that, but we don't. What's most important to me is that we all keep in touch so that if our kids want to establish a deeper relationship when they're older, if they want someone who is curious about their donor, too, they'll have those connections.
I have one set of friends who met (randomly at pride) and became friends because their kids share a donor. Now the parents refer to the kids as siblings and they are all thoroughly enmeshed in one another's lives. (the kids are 6,5,3) They have made a good connection with each other, and I hope it adds stability and continuity to their kids' lives, but I wouldn't introduce a child as a "sibling" when I couldn't control so many things- like their parents, the degree of involvement in each other's lives, the whole what-if-they-move-away thing, etc etc.