There are plenty of reasons why you may have trouble getting people to add you to a social network. First of all, you should take a look at who you're friending or following. If your list is made up of celebrities, high profile business people or companies using social networking to distribute press releases, don't expect many adds.
If you're following everyday people and you're still not getting reciprocal adds, it's time to look at what keeps people from following you. Because a 140-character Twitter update can be sent from a computer or a mobile device, it's very easy to send a tweet -- almost too easy. If you send tweets too much, your followers may sour and potential followers may not add you. When people look at their Twitter homepages, they want to see a mix of tweets from various people. If the majority of a user's homepage is made up of your tweets, that user may ditch you.
The opposite side of the coin is the person who never updates. No matter which social networking site you use, if you don't visit your page, don't expect anyone else to either. It may be tricky to come up with something interesting to post on a regular basis, but it's worth the effort. Users don't want to follow someone who only reposts news headlines or posts material that seems self-promotional or too introspective.
People become involved in social networks because, on some level, they want to build relationships. They're going to follow people who post interesting and relevant content and who interact, reply and share. If every post is a broadcast, you may come across as a know-it-all, and if every reply brings the topic back to you, people won't want you in their networks.
Finally, be patient. Even if you implement all of the strategies in this article, relationship building takes time. It's easy to think that in the fast-paced world of the Internet, things will be different, but they really aren't. It takes time to build a body of posts that people want to read. As you follow others and add more and more interesting and relevant content to your page, you'll find that more people begin to add and follow you.