I read about this in The Facebook Effect, a book by David Kirkpatrick. You can actually read the passage here [1] (beginning at the bottom paragraph of page 24

I read about this in The Facebook Effect, a book by David Kirkpatrick. You can actually read the passage here [1] (beginning at the bottom paragraph of page 241). Basically, Hotmail was blocking friend invites from Facebook altogether on some days, and on those days growth would drop by 70%. So, they cut a deal with Microsoft to unblock them. It stands to reason that they did this with at least some other providers as well, although at the time Hotmail was by far the most important roadblock that they successfully removed. [1] https://books.google.com/books?id=RRUkLhyGZVgC&pg=PA241&lpg=PA241 https://books.google.com/books?id=RRUkLhyGZVgC&pg=PA241&lpg=...