I think that mostly things on thumbtack just seem rather static, whereas craigslist is fluid. Craigslist is pretty close to pure UGC. On thumbtack the leads come in kind of slow (0-3 a month) and everyone competes with static profiles and a chance to write a short note (which is nice, but also can feel stale, and untimely)

I want to go to the website and connect with people ready to work together right now, or at least communicate with people in real-time. If you are active on craigslist and have the time you can email people until this happens, and it usually doesnt take that long.