The function user_profile_social_update_module_availability is called from an upgrade step. This includes this line:

$modules = $DB->get_records('course_modules');

If you have a large number of entries in the course_modules table, this is likely to use all your memory because:

• It loads all the records even though it only needs the ones with availability field set
• It reads all the records into memory at once (should use get_recordset which loads them in batches of 100,000 or so)
• It loads all the fields of the records even though it actually only needs the id and availability fields