Wow
That's venture capital for ya, but realize that he's invested similar amounts in lots of companies that he is yet to (and may never) see any return on...
Also, the it isn't $1bn, it's more
like $400m...
ETA: Also, when he invested in FB, they weren't accepting venture funding from anyone, as I describe below, venture funding is more complicated than that, and investors offer intangibles besides for the cash they front.
This book discusses a lot about what went on in FB's early days.