Friends who borrow your notes and share theirs back are partners. The ones who only borrow are tenants.

Exchange and extraction look identical for the first few weeks. Both involve somebody asking to borrow, and the difference only shows when you need something back: one person offers before you ask, the other is suddenly busy or has not kept up. The second is not always malice. Some people simply never think about the other direction, which amounts to the same thing for you.

Keep a loose count rather than a grudge. If you have lent notes four times and received none, ask directly for the next set and see what happens. And check your own ledger too, because it is easy to be a tenant without noticing.