I'm a lone developer, and frankly I never cared about social media for my personal life. But it seems indies need not only build good games/software, they also need good marketing.

This is true for any product, you cant just create something and hope it will sell. You need people to sell the product to who know that it exists. This is why the marketing budget for AAA titles is so large, usually much larger than development costs.

Do you use social media for promoting your game/software?

Yes, how else will people know about your game and what you are doing.

Do you use studio/company accounts or personal accounts?

I would use a studio account as this gains your studio recognition if you ever release a game under the same studio. ( Brand awareness )

Do you have a dedicated website and forums?

This is critical how else are like minded fans supposed to discuss your game with eachother. Also if you have nowhere for them to go to get updates or information on the game do you really expect them to stay interested while you develop said game.

How would you go about building a following/community?

I would suggest posting weekly blogs, and posting to facebook. Going to game dev meetups and meeting like minded indie devs will help get your projects out into the wild as well.

Would you agree that posting regular updates on facebook, twitter, instagram and what have you will take considerable effort and time from actual development?

Yes but its worth it. You can't sell a game to people who don't know it exists.

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I would recommend reading this its very informative and answers your questions;
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http://www.pixelprospector.com/the-marketing-guide-for-game-developers/

Edit: made easier to read

Thank you.

Do you think it's better to focus on marketing while the product is still in development or wait until it's released?

There is a middle territory where you should start marketing. No need to market until you have a solid product ready I would say an early alpha. Playable, but not all features added. This allows you to have something that excites the person looking into your game but also gives them info on whats to come.

If you wait until you have a finished product you don't have that community built that you are going to need to get the word out. But on the other hand if you get people hyped to early, the interest dies down before your product is ready to be released.

If you get a good community going they can help with design and you can kinda find the crowd you are catering to and see what they want as well.