"The hacker was able to view the names and Social Security numbers of 400 customers, all of whom were notified in writing about the break-in, T-Mobile said"
This quotation is taken from CNN site.
I was wondering why they use here "whom" and not "who"; "who were notifed" - "who" is the subject here. It's would suit here if it was an object, would't it? like here: "all of whom HE notifoed".
Do I miss anything? Thanks
The preposition "of" is the boss here!
After any preposition, "whom" is the word you want if it's a choice between "who" and "whom".

of whom, among whom, for whom, with whom, to whom, by whom, ........
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I had a question some time ago re: a similar issue.

In my case, the object of the preposition was actually a noun clause (something like "to whoever [or 'whomever'] solves the problem"). The "whom" form is usually the correct one, but the 'whoever' was the subject of the noun clause, which was itself the object of the preposition ( have I lost you yet?).
Right. That's a more advanced concept. The "whoever" / "whomever" choice works a little differently. In this case, the function of "whoever" / "whomever" within its own clause governs the choice, and the preceding preposition, if any, has nothing to do with the choice.

(I hope I haven't lost you now!)

Emotion: smile