Rule #1: Be nice. Or not. But anything you say can and will be held against you.
Posting on a social network like twitter or facebook is akin to chatting at a party or in a café. Be aware that others can hear what you’re saying. That’s the point of a social network. Act accordingly.
Assume others can and will repeat what they hear. To anyone. If you’re participating in a discussion with people in your professional field, behave as you would at an office cocktail party. Don’t say anything about someone else or a company that you wouldn’t say to their face. Example of twitter disaster by an ad agency, about 2 weeks ago.
Never assume that your conversation is completely private. There are occasional security breaches/errors on any network, so this rule also applies to anything you type/add, even with full privacy settings on.
That said, if you’re friendly, helpful and responsive, you have nothing to fear. Enjoy your new contacts.