I know there are many websites I can join, but I doubt who have so much time update content everyday.

I have seen some courses teach web 2.0 marketing, but how can I have so much time to join and update many sites and do it in correct way?

These courses all offer tons of tips, strategies, but who can really apply those strategies?

Let's start with the basics: what are you trying to achieve?

"web 2.0 marketing" you forgot to add the BS after that statement

How many social networking sites should I join?

somewhere between NONE and all of them.

I have seen some courses teach web 2.0 marketing, but how can I have so much time to join and update many sites and do it in correct way

Or then again you could do something useful with your time.

These courses all offer tons of tips, strategies, but who can really apply those strategies?

Sooo! when do we see the useful link to these "tips and strategies" that someone will leave "specially for you", or am I being cynical again?

Perhaps, before joining such sites, you should spend some time visiting them.

I think you will find that they are not very receptive to pyramid marketing schemes, and your answer will be zero.

I would advise only a handful

For social networking sites to help you generate traffic, you need to build a reputation on them

This requires an investment of time on each site, be it facebook or myspace.

So find the social networking sites that are relevent to your niche and have loads of traffic. Pick a couple to start with and you will soon be able to determine how much time you can afford to spend on them and what other sites you should join

I suggest should join the ones your closest friends are on. You can limit yourself to one hour per day per social network.

It depends on what kind of site you're trying, social networking doesnt work very well for all kinds of website