You're wanting to update a field when the user saves activity? That's doable with the trigger activity_onAfterCreateStreamActivity, but it's highly unlikely to work out how you're likely expecting it too. Most activity is generated by system actions and doesn't even store a title or message. Activity posted by the user also doesn't have to have a message if it includes a photo, link, location, or action. The below would probably work, but isn't entirely accurate since it's only checking for new status posts if they contain a message.

Global
Triggers: activity_onAfterCreateStreamActivity
Type: Field
User: Automatic
Access: Everybody
Conditions
1: [var3_asset] Is REGEX /^profile\.[user_id]$/
2: [var3_user_id] Equal To [user_id]
3: [var3_message] Not Empty
Action
Field: YOUR_FIELD
Operator: Set
Value: [var3_message]