Each time you search for something on Facebook it gets automatically saved to your search history in Facebook activity log. Saving your Facebook history is helpful for you. Because it makes easier what you are looking for. But whenever you search on Facebook all information you searched get gathered in background by Facebook. So Facebook search history is way of Facebook to collect information about you and Facebook can use such kind of information for advertisement. So what to do? Don’t be worry you can delete your search history on Facebook. Here’s how you can erase your Facebook search history:
1. Log into your Facebook account Click 
at the top right of Facebook and select Activity Log from drop-down menu.
2. You will see list of options displays on the left side of the Activity Log screen.From that list click on More under comments to expand list. 
3. Now choose Search from expanded list.
4. By clicking on search option your all Facebook search history will display by date.To delete one search item, click the block icon to the right of the item and select Delete from the drop-down menu.
5. by clicking on delete option remove Search dialog box will appear on your screen for making making sure you want to remove the search. Click Remove Search to complete the removal process and that’s it. This way you can delete your Facebook history day by day and of course it will take time. So if you want to save your time you can delete your all Facebook history at one time and it will take just one more step.
6. To clear all of your search history, click the Clear Searches link at the top of the page.
7. Here also a confirmation dialog box will appear for this action,and the dialog box reminding you that your search history helps them show you better results when you search. What they don’t say is that it also helps them gather information about you. So click Clear Searches to remove you all search history.
thus you can delete your entire Facebook Search History. It’s good for you if you often erase your Facebook search history because by this way you can restrict Facebook from gathering more information about you.
Also Read: How to block people on Facebook & How do you unblock people on Facebook