SQL DB design to support user feeds (in application like facebook)
Get rid of ambiguous columns like object_id.  Use log_type to determine what you are querying on.  So something like:

CREATE TABLE logs (
    log_id bigint AUTO_INCREMENT, -- not sure if this is the mysql syntax
    user_id int references users(id),
    post_id bigint references posts(id)
);


A second point is I would suggest changing the friend/unfriend to be immediate so it doesn't hit this table.  This should be only for logging actions to posts, not friendships and the like.  If you must log and propagate, create a separate table for this.