I use Twitter more for making connections with people as it is much easier to reach people on that than Instagram... at least that's been my luck. You can promote your stuff on Instagram but it's probably a bit annoying. I do make a post on Instagram with either a thumbnail photo or a short clip from the video. I think the most important thing about these platforms however is hashtags. You never know when people will be searching through hashtags. I know I'm not one who really searches through them much but there are people out there that just follow certain tags. I did some E3 videos where I tagged a lot of the games announced during this year. I got a lot more random likes on the photos but I don't know if that translated to views. But hey, at least I know people were seeing the posts!
Don't be too spammy with promoting though, that can really make people want to unfollow you quickly. I know. I follow some accounts that just do it too much and have been very tempted to unfollow them because of it.