Half of all friends replaced every 7 years.

And how does one find these... "friends"? From Yahoo! News:

You may have more Facebook friends as the years go by, but when it comes to your close friends, you lose about half and replace them with new ones after about seven years, new social research suggests.

As a result, the size of your social network stays about the same.

People might like to think they have control over whom they choose as friends, but social networks could also be influenced by the context in which we meet one another. Sociologist Gerald Mollenhorst of Utrecht University in the Netherlands was interested in finding out exactly how much our networks are shaped by social context or by personal preference.

He conducted a survey of 1,007 people ages 18 to 65, and then contacted the participants seven years later. From the original group, 604 people were re-interviewed. The survey contained questions such as: Who do you talk with, regarding personal issues? Who helps you with DIY in your home? Who do you pop by to see? Where did you get to know that person? And where do you meet that person now?

The results showed that personal network sizes remained stable, but that many members of the network were new. About 30 percent of discussion partners and practical helpers had the same position in a typical subject's network seven years later. And only 48 percent were still part of the network. This finding goes against previous research which had showed that social network sizes are shrinking.