I understand Saturday is sabath but mi.yodeya is a site for jewish learning and nothing related to work , so why do people disappear from here on Saturdays? Does answering questions or questing etc also considered work?
As noted in the comments, using electricity is melacha and thus forbidden on Shabbat. See this question for more information about why electricity is prohibited. In addition, writing is forbidden on Shabbat, even if the text is ephemeral. Since, so far, it is not possible to ask and answer questions on Mi Yodeya without using electricity and writing, the Shabbat-observant participants here do not engage in those activities on Shabbat. (Or Yom Tov, holidays.)
Shabbat is of course determined by local astronomical events and Mi Yodeya is a worldwide community -- just because it's Shabbat in North America doesn't mean it is in Australia or Israel or Europe, after all. Further, as noted by Daniel in a comment, this site is also used by non-Jews, who aren't bound by Shabbat restrictions and are welcome to use the site then. So you will likely see some activity here on Saturday, but less than the rest of the week.