Single site is enough for promotion purpose rather than different sites. Sub domains are good but for all purpose these are not good. Create folders and place your specific information over there. Your site will convert in Authority site which will give more
weightage to your website.

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Live Chat Software | Free Instant Messenger | Pick Baby Names

both things work fine but i suggest folders work great in comparison with sub domains.

Also it depends what is your business models some times sub doamins also work good as google has all sub doamins, if you have more content in same niche then a big site is good and if you have various business models then multiple sites are work fine as those sites are easy to maintain.

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SEO Services India | GrabRanking.com

If you manage to manage multiple sites ... go for it. If not only big one. Also for multiple sites ... try at first sub-domains: it's cheaper then buying new name domains.

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Coupon Site Script - Turnkey Affiliate Moneymaking Business

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I recommend you to host a single website with required sub-domains. Your website will become an authoritative website and search engines give priority to it. One more thing.. It will be easy to handle all stuffs at the same time.

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Live Chat Software | Free Instant Messenger | Pick Baby Names

My opinion is even if we use sub-domains... search engines consider subdomains as part of the main domains and not a different site altogether. if you search site:sitename.com... you will be shown all the subdomains also.

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Unsecured Loans for Unemployed and on benefits | Get text loans from reputed payday text loans lenders in the UK

I would go with multiple sites.

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Free Search Engine Submission
Website Promotion Services

I think it is easy to manage one site rather dividing your time & mind to different sites.

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