Well for now I use twitter and google plus (also I upload some video on reddit in the r/letsplayvideos subreddit because I can't find specific subreddits that allow pure let's play videos). I don't know if this is really helping me getting views and actually I don't like to promote myself too much because I don't want to become an annoying spammer so for this week i'm trying this: I will only send the automatic message during the upload on G+ and twitter (of course using hashtags) and I want to see if it can create a sort of difference.
Anyway I think that twitter can be really useful but you can't just use the account for link dumping, just use it like other social media (find other people, chat with them) and I think that you can obtain results. Also I don't know if instagram can really be useful for promoting but maybe that is only my opinion.