First, it is rarely useful to look at huge sites like about etc; separate rules often apply.

There was a recent fad for subdomains, which seems to be passing (unless users just aren't talking about it).

In general, SEs treat subdomains exactly like they'd treat any other domain - so a site with four subdomains appears as a bunch of five sites.

Immediately there are risks, especially if all the sites are swapping links with yet more sites.

You are also dividing your marketing, between five sites, your incoming links between five sites ... your ranking between five sites.

Not to mention visitor confusion.

Advantages? I cannot see one! If a bunch of content deserves a site of its own, by all means. If not, use a folder.

If a bunch of content deserves a site of its own, by all means. If not, use a folder.

Thanks, this makes me more confuse on use of subdomains, so when and where exactly should we use sub-domains in a website?

One can find new sub-domains rising everyday for google/yahoo/msn etc.....

Do SEs bypass this rule for themselves?

For huge sites, on many servers, there are good organizational and probably good technical reasons for using subdomains.

For 'the average webmaster', there are virtually no good reasons to use them - and plenty of reasons not to.

And from an SEO point of view, they are a disaster. (Been there, still wearing the T-shirt!)