Simplest answer: twitter is where everyone else is, which makes it the simplest and easiest way to interact conversationally with potentially anyone on earth without an intro or in-person meeting.
In addition, twitter enables easy one-to-many communication for celebrities who want to spread memes, and brevity is the soul of wit.
If you want to explore the community more, I recommend reading the Library, checking recent Curated posts, seeing if there are any meetups in your area, and checking out the Getting Started section of the LessWrong FAQ.
The Open Thread sequence is here.