I've been using LinkedIn as a way to connect with industry colleagues for the last few years. It's not a site I visit as often as Facebook or Twitter, but it definitely does have its own unique value for business networking.
I'm always happy to accept invitations to connect with legit accounts. Recently though, I've noticed an increase of "mystery profiles". That is, an account with a name, but no photo. I'm not sure if these are spammers, or just photographers who are too lazy to upload an avatar. If you're trying to advertise yourself as a photographer, wouldn't this be the first thing you do upon signing up? If there's no photo, I have no choice but to click "ignore".