The "Friends" function is actually a massive database being compiled by a third party resource. When you request or accept someone as a friend, you inadvertently agree to a digital contract to become that person's godparent and they become yours. If requested, you also are indebted to them for at least one "big favor". In all fairness, if nothing has been requested from you (or by you) before severing the friendship, no penalties are applied and no further communication between yourself and and the former friend is required. The third party database is purged after 12 year from inception. However, backups of that database may linger for up to 25 years.
I think it gets a little more technical, but I'm not sure.