I can understand your concern; initially when I went onto LinkedIn I was being pestered a little bit, but mostly by recruitment agents as opposed to directly by companies. These were typically based on keyword searches and the like, and their approaches had all the subtlety of a Nigerian scam letter or a Reader's Digest "You are our lucky winner" circular.
From the employers side of things, I have approached one person to join my team based on their online presence - actually a Wiki they were maintaining to support users on their site with our software - and the individual was an excellent choice, even though we had to relocate them internationally.
To take this further, when someone applies for a job with me, I expect them to be able to explain why they want this job (and not just a job) If the prospective employer approaches you, I think its more than fair to invert this expectation, and explain why they want you for the role.
If they are responding to your wider online footprint as opposed to just a profile then - even if it is an recruitment agent - I'd take it seriously.
I've had experience of companies passing a targeted name list onto a recruitment agent based on just this, as they wanted the initial screening to be anonymous.
So - I'd suggest if its not obvious from the initial contact, screen the approaches in exactly the same way as companies screen applicants for roles they have advertised. Most genuine employers would see this as a positive step.