The publication of their email address means that you have an opportunity to initiate contact and build a relationship. That sort of availability simply doesn't exist in traditional networking. Managing your circle of contacts is greatly simplified. You can use contact lists that are internal to many sites, or your own electronic address book to manage your growing circle of E-Networking contacts. There is no need to collect numerous business cards with hastily scrawled reminders about each individual.
Many sites sponsor networking circles and events.
Some of the best E-Networking sites are traveling throughout the country, hosting presentations and seminars for their members. The assumption is that you have met colleagues virtually, and now want to cement those relationships at an evening dedicated to further networking and perhaps even professional development. It is important to cast a wide net when you are networking on the web, simply because you do not have a personal introduction to pave the way for you. Sometimes it is difficult to tell, by the information that is first available to you, if someone will be useful as a networking partner. So do make your initial efforts large, and then scale back.