A United States-based analyst says Twitter will almost certainly make changes to its trademark 140-character limit once it is listed on the share market.

Twitter has revealed plans to raise $US1 billion through an initial public offering.

The chief executive of financial information provider PrivCo Sam Hamadeh says both Google and Facebook have increased advertising on their sites to justify their market valuation.

He believes Twitter will also need to be flexible to satisfy its shareholders.

"Once it's a public company, it has very demanding stockholders to meet and missing earnings or growth slowing - Wall Street has no mercy," Mr Hamadeh said.

"And so you can bet they will punish Twitter for any misses and I think it's inevitable they will loosen that 140-character limit. It's not a sacred cow."