Twitter hashtags are great, but sometimes it's hard to track conversations around a tag simply by using Twitter search.
That's where new service fTag comes along...it attempts to replace the hashtag with an "ftag" (a word with an underscore before it) and pulls together all those tagged Tweets into a single stream on the site. It also lets you contribute to the stream from the fTags site itself. So whether you want to talk about Lost Dogs, or Global Warming, or Mashable, fTags provides a place to do it.
It's a neat idea with one (potentially big) vulnerability: the obvious next step for Twitter to challenge Facebook would be to launch Twitter Groups, single pages built around hashtags. Twitter hasn't been adding these obvious features due to issues simply keeping the site online, but if they did make the addition, fTag and the many similar products around would likely disappear.