Before the days of social networking, hackers and malicious types had to rely on email to distribute their evilness. They'd send an email with an infected attachment of something that looks interesting that are designed to tempt you in opening the file. Things like "Cat does handstand on top of dog's head", "Naked Man with big tennis balls at Wimbledon" or "Cute girl on Channel 4 Countdown spells rude word" - most people caught onto the fact that a lot of these unsolicited emails will most likely contain a virus meaning that the trust in email attachments has been lost and therefore the possibilities of attack via email becomes much less.
Moving into the 21st century and the overwhelmingly popularity of social networking sites, such as facebook and twitter, the malicious hacker is provided a whole new and far more powerful means of infecting hundreds, thousands or even millions of people - the reason is TRUST.
The whole point of a social network is to set up connections of people - friends, families and celebrities and then things like pictures and websites links are shared within those connections. It's fantastic for celebrities as they are able to keep their fans up to date with their celeb lifestyles and latest projects.
The fact that someone trusts the people that they follow is hugely valuable to a hacker wishing to infect a large amount of people. Just as with email, if someone unknown contacts you via twitter asking for money, then you know its a scam. Whereas if a friend or work colleague contacts you asking for money then it appears a lot more legitimate. If a friend sends you a link to a website claiming to have a picture of you at the office Christmas Party looking drunk while sat on the photocopier then its more likely to be opened as its from a "trusted source".
This trusted connectivity between friends and family is the reason why sites like facebook have become the new battle ground in spreading computer viruses. The reason is as above, people are more likely to open website links and attachments from those they know - if a hacker can gain access to one person's profile, they can obtain a lot of personal information for things like ID fraud and then infect the connected friends - as the request with infected file \ website will have come from that "trusted source"
As an example of celebrity popularity, Carol Vorderman has over 250 thousand followers on Twitter, Rachel Riley has 50 thousand, Richard Branson has 2 million 700 thousand, Stephen Fry has 5 million and Barack Obama has 25 and a half million followers. Hackers and the malicious type would probably give their right arm and leg to breach a celebrity's social network as it would mean that they could potentially infect thousands of people across the globe with one single message.
It's also worth keeping in mind that a friend, family member or celebrity could cause a security problem by accident - lets say they find a really interesting site which they then tell their followers who all look at the same website - the problem comes if that website just happens to have a hidden virus that infects all their followers as well.
Obviously its not easy in today's world to ignore the internet and stop the enjoyment of sharing fascinating finds with friends and followers.