I do a lot of Social Media strategy and statistical analysis. Here's a set of tips about the major social networks hashtags.
First of all, let's get this out of the way. Hashtags WORK. On everything. To a greater or lesser degree.
Instagram. Hashtags are equally important, but the audience is fairly different. On instagram, tags involving food, selfies, and art will get the major response that you're looking for. When hashtagging #java on twitter versus instagram, you will get a much different response.
Facebook. Hashtags are nearly completely irrelevant here. In fact studies have shown that using hashtags on personal posts discourages people from looking at your content. Which is a shame, because Facebook is set up to handle hashtags, it's just that no one cares.
Google+. Sigh. Why won't they just release an api so that third party apps can post to them? Google Plus is unique in that they will auto tag your posts if they find relevant tag information within your post. You don't even have to try. Of course, you can also help it along with some actual tags of your own.
Tumblr. Hashtags get generated as a side content for tumblr posts, and so don't have to be a part of your content strategy, but can be tacked on as meta information. This is a nice addition to the process. Tumblr thrives on immediate image content, and so photography and graphic tags tend to get the most play.
Is this a comprehensive list? Not even a little. These are just some notes I've made along the way while trying to stay relevant in a world that moves quickly every day. Hopefully you find something useful in here.