This is an older question, but as this seems to have come into effect last night it's probably worthy of an update.
When filling in the DS-160 form to apply for a US non-immigrant visa the following question is now included :
The list of available Social Media platforms is :
As to whether you should provide this information and what happens if you don't, the simple fact is that by not providing this information would be you entering false details on a visa application, which could very likely be treated as making a fraudulent application. That could have consequences up to and including your visa not being issued, or being revoked at a later stage. Obviously how likely this is to occur is unknown as this stage.
As with all questions on a visa application the best advice would be to answer it honestly - although even then there's signifciant grey area here as far as what constitutes a "social media account" (eg, I have a Google account, and I used YouTube - but does that mean I need to list YouTube even though I've never posted any videos on it?)
Note that this is unrelated to the similar question on the ESTA application which is optional so can be skipped.