On Twitter, I have some personal preferences about who I follow. When someone follows me, I usually do not follow them back directly for some reasons, mostly it’s because of spammy users. If someone starts following me, I usually check his/her profile first. I tried not to follow people back without reading about their profile (and their updates).
I will not follow other Twitter users back mainly because of these two reasons:
I do not know them. The word “know” here is not about I have met them in real life. I look up their profile. If they have no profile, website/blog URL, or short biography. I will not follow them. I might block them.
They follow many people, but have less follower and updates, e.g. Following 800+ users, less than 10 followers, and the first update is few minutes ago?
Usually, I block them right away. Anyway, I follow other people not because I want them to follow me back.