they could develop new features but intentionally implement them in a way that they are not compatible with other services. they could put all the other instances they federate with on rolling blackouts so that it seems like they are down when in fact it's just them cutting the connection. doing just these two things with purpose could make it look like Facebook has the most advanced and stable instance. in addition, as you mentioned, it would also have the biggest populace. there would be pressure to abandon other instances to join that instance to stay in touch.