Strong increase at the start, then a relatively quick drop, slowly declining to un-hyped levels of participation. This isn't news, this is common.
Strong initial growth is usually based on hype. There was tons of hype. The user count dropping so quickly means that the people, who joined because of hype (whatever sub-reason covered by the term), are leaving.
Given that it's facebook, the hype and potential participation rate was gigantic. It's a bubble. Bubbles pop.
More news at 11.