Most people use Twitter for personal purposes. Some people I know still don't see the value in using it for marketing purposes, but in reality it's a very powerful marketing tool and a great way to make money.
Consider these 2 ways to make money with Twitter.
First off, you have sponsored tweets. If you have a large Twitter following than other marketers or companies can approach you with the option of your putting out a sponsored tweets. All this means is that you or someone else writes a sales driven Tweet and then all you have to do is send out that Tweet to all of her followers so that whoever paid you to send out that Tweet gets their message out about their company or website or product. Some companies may approach you to do this multiple times and the larger your following, the more money which you will make because obviously the more people who will receive your sponsored Tweet.
There are even services which enable advertisers to freely access your Twitter account to send out regular sponsored tweets so that you don't have to lift a finger and they even handle your payments for you so that you receive regular payments from that advertiser.
Conversely, you can market your own products or someone else's products as an affiliate through Twitter. Affiliate marketing is the greatest way to make money online, and tweeting out links to reviews of products or services which you have used and recommend to your Twitter following with your affiliate link included can turn into a huge paycheck for you.
There have been stories of some of the biggest marketing gurus out there recommending a new product through one tweet sent out to their massive following and earning 6 and 7 figures from that one tweet. This is certainly incentive to develop a huge Twitter following.
Remember, though, that no one is following you on Twitter to be sold to. You need to intersperse your sales driven tweets with lots of other content. This includes conversational tweets with other Twitter users, quality content tweets and tweets with links to quality non selling content, and it even helps to tweet some personal, light hearted, and humorous tweets from time to time.
Other Twitter users like to see that you are a normal person like everyone else, and conversational and informal tweets can really help to humanize you in other people's eyes.
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