The same goes for the follow by email widget. If they can't find it, most people won't go looking for it.
Put your social media buttons and following options somewhere easy to find.
The sidebar isn't the only option for the placement of social media buttons, you could also put them in your blog footer or in your navigation bar. I've even seen a couple in the blog header.
It's also important to make sure there's a way for people to follow you through your mobile site. If you want people to follow you, you shouldn't make them look for a way. Now more people then ever are browsing the internet on a mobile device, these days a large portion of my views come from mobile devices.
Have a couple different options to follow your blog.
For example I have email subscription and bloglovin. I would recommend to have at least these two, but I'm sure there's probably a few more options out there you might want to consider, such as RSS feed, and I've seen a lot of google connect (though that only works on blogger blogs now)
Some people may prefer to follow you one way and others another way. So having some different options would be a good idea.
Just remember if you want to gain followers you'll have to make it easy for them to follow you.
Now this doesn't mean you should shove following options in their face. It just means the options should be easy to find.