Spammers find it easy to succeed on sites like LinkedIn and Twitter.  It is easy for them to find a target market based on what groups the target audience belongs to or what they are saying.

This is unfortunate as Twitter can be advantageous to those who are trying to use the sites to connect and communicate without being pitched to.

Be cautious as to where you post your email address, as spamming laws were limiting the spam for a while, but if you interact with someone online or belong to the same group, the spammer may use this as a license to communicate with you.