Are you sure the data is not being added? It appears that your function is firing. Do you mean to say instead that the saved data does not get displayed after the function has fired?
There is nothing in the few lines of code you’ve pasted above that would make your data_to_add() work when hooked to the wp action event but not the bp_activity_posted_update action event. Since both events get fired and subsequently fire your added action function, the data should be written to the DB in either case.
