So I was on Model Mayhem today, had a cold, and was posting a bit in the forums. Don't get me wrong, the site has helped me out, it helps others out, and it's not a bad place to be on (agency models should talk to their agents first though). But, the problem is when a model gets into the "testing trap" and wastes time online when they should be knocking doors at agencies.
So just off the top of my head, and again I have a cold, here are some issues with those sites in general and why it may not be a great idea to join right away if you want to model.
The Testing Trap
If you want to model, and are around industry standard heights and sizes, then you would be better off talking with agencies first. Online, at modeling sites, there seems to be this perception that "models" need to always test, and be "testing up", to get real world work. This isn't really correct. No one in the industry cares, agents don't care, and it may even hurt you later.
At best, it may, eventually, after a while, get some images that may kind of work for real world modeling.
At worst it will waste a lot of time, get terrible images, or get images that may put off clients later when they want to hire you.
I feel, rather than mess around on these networking sites if you're a industry standard size model, that it's better to talk with agencies first or a local professional photographer first and pay for a shoot - it's more expensive, but it gets results a lot faster.
Google has an image search
I looked up a model there today on google, and lot of the images on Mayhem were listed under the models name as "lists" which will be very hard to delete later if you're looking for work. I'm not saying this is a privacy issue, that it's bad, but it happens and it's something to be aware of.
If a client were to hire you, did a search online (which they do) those images may come up. Now this may or may not hurt you, but at that point it's a little out of your control.
There are more, and I may visit this later, but these are just off the top of my head.