Is a divorce effective if one says, "If you open Facebook in my presence, you are divorced," and what is the ruling on reconciliation after forgetting the oath?

If the wife does the thing sworn upon, forgetting the oath, then the husband does not break his oath. And the husband's utterance of "raj'ah" (resumption of marital relations) as a precaution does not result in divorce. The preponderant opinion is that intention is considered in an oath. So, if you intended to prevent your wife from opening Facebook for a specific period, your oath is restricted to that period, and you do not break your oath if she does it outside of that period.