When I search for "facebook" in Sender (whether using basic or advanced search) it finds most, but not all of the messages.
Here is the From: header from 2 emails it does NOT find:
From : Facebook <notification+xNNNNNN@facebookmail.com>
From : Facebook <update+xNNNNNN@facebookmail.com>
Note that it finds most other emails with this exact From header.
And let me say, that searching is really the ONLY significant advantage gmail has over zoho. Please make search better and then I can recommend zoho to my friends and colleagues without reservation.