Last activity date incorrect

Hi,

I've created a rollup field for the last activity date on a Lead and Opportunity, but it shows only the last date from the Social Pane and not other fields (e.g. Modified On)

Do I have to create a workflow for the Last activity date field to fetch the date from the Modified On field? Or how would I ensure that the Last activity date field would show only the last activity done on the specific Lead or Opportunity?

Thank you!