You may have noticed over the last day or so that Facebook is now displaying the most trending topics within its network. The concept of trending topics is very much intrinsically a Twitter one, and is closely tied to the proliferation of the hash-tag.
By copying such a fundamental part of twitter, they are diluting the uniqueness of Facebook's offering. Facebook's entire premise is around organising the web around people and their connections, not external topics.
So why is Facebook going down this road? Of course the answer is the same answer that governs 95% of all digital marketing platform decision making - money! Twitter has done a great job monetising their "promoted trends" product and has been doing it for many years.
The question now is, how will Facebook monetise their trends, and allow advertisers to gain access to core inventory on peoples timelines. Will it be a simple algorithm which calculates the speed/volume of conversation on a topic, or will they weave in demographic elements based on the users profile so sponsored trends are more relevant. If they can, they may find themselves ahead of twitters promoted trends product, especially since they can win on scale of inventory alone. Half of Twitters conversation and audience is entertainment or celebrity related. Facebook has a far larger user base that touches on every vertical and demographic across most of the globe.