This is how i solved this problem

Download your APK to your PC in java jdk\bin folder

bin folder might look like this

C:\Program Files\Java\jdk1.7.0_121\bin

go to java jdk\bin folder and run cmd then
copy the following command in your cmd

keytool -list -printcert -jarfile yourapkname.apk

Copy the SHA1 value, which looks like this

CD:A1:EA:A3:5C:5C:68:FB:FA:0A:6B:E5:5A:72:64:DD:26:8D:44:84

and open http://tomeko.net/online_tools/hex_to_base64.php to convert your SHA1 value to base64.
This is what Facebook requires. Copy base64 value and paste it in key hashes under settings->Basic -> Key hashes