Facebook is testing new summary emails for users who are very active on Facebook and receive a lot of emails. Instead of receive individual emails for each notification, which can get obnoxious for any service, the new test rolls multiple notifications into one email, and sends out occasional digests of compiled notifications instead.
Facebook is automatically adding users who generally receive a lot of email notifications into the test, and is automatically changing their account settings to shut off most email notifications. Facebook defaults to sending users emails for over 70 different types of notifications on their service, from receiving personal messages to being invited to Events to being a part of a frequently updated Group, so power users may actually find the summary emails a helpful way to reduce email clutter, though some users may not be happy with the digests and prefer the individual emails as an easier way to sift through the important stuff. If you happen to find that you have been automatically opted-in to the change, you can go back to the normal, digest-free option by popping into your account settings and unchecking the “Email Frequency” option.