Borrowing from its rival should help keep Facebook relevant.The trend of major social networks borrowing key features off each other is nothing new: the Facebook ticker that sits alongside the news feed is very Twitter-like, while Twitter has introduced Instagram-style filters into the photo capabilities of its mobile apps. Facebook's adoption of Twitter's hashtag feature may ruffle a few feathers, but (like Google+) Facebook had to adopt it eventually or risk being left behind.
The appeal of the hashtag lies in its simplicity -- it's really just an extension of the search capabilities already built into Facebook and Twitter, but it makes them far more immediate and easy to use. You can already search public posts on Facebook for certain words, and as the super-fast Graph Search