Stack Exchange network consists of 177 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Area51 visits/day count is different from main site. In fact, it changed from the last time this question was raised, but then the number on Area51 was lower, and now it's been higher for some days now (1478/day vs 1540/day).
Main site uses a gross average. Area 51 pulls directly from Google Analytics so it is more correct.
See my answer on meta.SO.