Social Media Analysis (SMA) is the easiest way to see who's not following you back.
Simply enter your twitter handle/username (or any Twitter user's username) and instantly get back the list of the accounts that user follows, but is not being followed back by.
This makes it easy to identify and unfollow accounts that no longer follow you, as well as periodically identify who may have unfollowed you.
SMA can also help you find the reverse info, who your not following back (or any Twitter user), but is following you.
Just enter any Twitter username and instantly receive a list back of the followers that aren't being followed back.
This can be super helpful when trying to grow your followers list as people who don't follow back typically unfollow you, so you can find them before it happens and make sure to follow them back.
SMA offers the best way to track your followers over time.
You can recieve weekly updates on all your new followers, and everyone who unfollowed you.
This is very valuable as you try to nurture and engage your audience by idenitfying who your new followers are, and reaching out to them proactively to further engage them and cement your relationship with them.
On the flip side it also helps you get rid of those nasty followers who unfollowed you and you no longer wish to interact with.
Another cool feature from SMA is user search.
You can type the name, username or even other information into the search field and find all the twitter users that apply.
This is the only way to interact with Twitter without needing to create an account and login. It is completely anonymous.
You can also search a user by User ID, or vice-versa, get their User ID from their Twitter handle.
This can be helpful if you're looking to use the Twitter API, or directly access certain accounts, tweets, etc. by a user ID.
Since the User ID never changes, but the Twitter handle can change, its the only constant way to refer to a Twitter account, that won't break over time if a user changes their Twitter handle.