#_social_notifications is too big in DataBase

Hello. In my DataBase #_social_notifications is 2+ Gb. It's too much.
How can I make it less?

I already set this option:
General settings > Notifications > Automated Cleanup

Enable Automated Cleanup: ON
Cleanup Notifications Older Than: 3 months

it is the lowest value.

How can I be sure that cronjob works correctly? It success convert videos, send emails etc.

Thanks.