In virtually every letter of recommendation I've seen, the final line is something to the effect of "Please feel free to contact me if you have any other questions".
But how often is it to actually get contacted? I've never actually received any follow up before, but virtually all the students I write letters for have gotten into the programs of their choice.
Granted, I only tend to write letters for undergraduates applying for scholarships and graduate school (no graduate program for my department) so maybe the likelihood increases for, say, letters written for master's students applying for PhD programs, but I'm curious if this is just happenstance that I've not been contacted or more par the course. (I know we do contact letter writers once we have a short(er) list for job candidates, but I'm thinking here specifically letters written for students who will continue to be students.)