At least at the agency we are working with there are no hard and fast answers. They encourage you to communicate at least 1-2 times per year (both sides, i.e. birthmom writes adoptive family and vice-versa, or they talk, or whatever) but allow each party (adoptive/birth) to set their own system up. While an agreement you reach isn't legally binding (i.e. if you promise to write the BM four times/year and then don't, there's really nothing anyone can do about it), they encourage people to reach reasonable and fair agreements that they are seriously committed to obliging because, obviously, that's kind of the point of open adoption.
Interestingly, they say that as time goes by the adoptive parents, who were usually leery of agreeing to a lot of contact, wish for more contact and the birthparents generally gradually drift away. It's partly due to the phenomenon you describe...many birthparents are young and in an unsettled place in their lives, and often the path they eventually find isn't one that has much time/space for a child they placed for adoption.
My DH felt the same way yours did (and still does to some extent), especially b/c the adoptive mom we heard from when we went to the introductory session our agency puts on has established a relationship with the birth-grandmother and sees her weekly (or maybe every other week). But as I pointed out to my DH that was this adoptive mom's choice and not something we had to do. We are also a bit leery of the whole extended family thing (even me) because honestly we have a hard enough time keeping up with our own large extended families never mind adding another one.
I'm sure the adoptive couple who were matched with my cousin were leery of this issue as they lived in the same neighborhood as she did at the time (no joke), and she was in a pretty unstable point in her life. However, per the above description she did what many birth parents do (I think) -- grow up, move away, she is now a mom to 3 sons she is raising. I saw her and her kids and the adoptive family and their kids (who now number 3, two adopted and one genetic conceived after years of IF and adopting the other 2 -- i.e. the stereotype!) recently at a big family gathering but I probably haven't seen them at such an event in 5 years (it was cool to see them, though).
I went to one support group mtg. at my agency which was interesting because a couple there who had just adopted #2 from a birthmom who didn't want much post-adoption contact, and they were not totally happy about that especially as they were worrying how their 2 (adopted) kids would feel if one had contact with his birthmom and the other did not.
Well, this is kind of rambly but I hope there's some useful info. in here somewhere.
ps I'm glad you enjoyed the Nate story...I did not know that about "Nate the Great," and I don't know Jenna but I might still post a comment about that on her blog!