Back in the summer I "friended" a young man from our neighborhood on Facebook.
I won't be available for the next few days to take care of the cats, so I asked this young man to fill in for me. Over the last few years, he's been our default cat sitter, and he does a great job. When I talked to him earlier in the week he agreed to do it; he informed me he still has a key from the last time. Because I didn't need to drop off any instructions, last night I realized that I hadn't given him my cell phone number in case he has any questions or problems.
I didn't have his cell phone number to pass on the information, so I did the next best thing. I left him a message on Facebook giving him the information. He seems to only check the page sporadically, so I called his parent's number (which I do have), and asked them to pass on the information.
Hopefully all my work will be superfluous and he won't need the number.