I've my various reasons for following, not following, or stopped following (replace "following" with "to circles" or "friended" where applicable). As much as I like to follow you back, I don't want my timeline to be filled with junk too. What is listed here mostly applies to twitter.
Reasons for following:
- You are already being followed by several people I'm already following and possibly receiving mentions from the said person regularly
- Your tweets are interesting
- Out of curisosity
- Tweets are mostly in a language that I don't understand
- Too much links, including "I favourited a Youtube video" or "MyAnimeList updated".
- Protected account, so I don't know what you are tweeting about (protected accounts you see in my following list were not protected at the time I followed them)
- Not interested / Never seen you before
- Too many retweets
- Boring tweets
- Ugly profile and/or display image (if viewing via the PC web version at that time)
- Has a #teamfollowback hashtag somewhere
- Number of followers is more than several thousand
- (For accounts of well-known people) Not from a personal point of view
- Adopting a wait-and-see approach if not having the same interest or not having anyone I'm following following them too.
- Tweet too frequently and clogging up my timeline, especially when I'm following a lot of people.
- Became uninterested. "Protected" accounts are not excluded.
- Spamming my timeline with rubbish at the time I saw you doing it. This includes replies to those that we both follow.
- Thanking all followers individually for following and not placing the username first. One right after the other for countless times. (Example)
- Not on good terms (They usually unfollow me first.)
- Tweets too infrequently (eg. 5 tweets a month), or the last one was a long time ago
- Changes to a display image (and maybe display name too) to something I find disturbing
- Obvious spammer (and gets reported too)
- Corporate accounts following me based on what I said recently
- Accidentally did so