BuddyPress.org

There’s a function you can use to do that called bp_activity_add(). See here for the usage:

bp_activity_add()

@bhekzin you’ll need to decide if you’re going to add an ad after X number of items have been added to the stream or if you’re going to add an ad every X number of seconds (or minutes, or hours). If you choose the interval approach, check out wp_schedule_event

https://codex.wordpress.org/Function_Reference/wp_schedule_event

@bhekzin actually, instead of adding the ad as an activity item, you could just modify the activity loop to insert the ad dynamically on every 8th item. This is how you’d do that using a for loop:

for ( $i = 1; $i < 1000; $i++ ) {
    if ( $i % 8 == 0 ) {
       // this is the 8th item so insert your ad code
    }  
}

Were you able to solve your issue? I think there is a plugin for this. Try a Google search for adsense in BuddyPress activity stream etc.