According to CBP official website FAQ on the Visa Waiver Program Improvement and Terrorist Travel Prevention Act Frequently Asked Questions, the question is completely optional.
Do I have to answer the new questions if the provisions of the new Act
do not apply to me?
Yes. All ESTA applicants must complete all
non-optional fields on the ESTA application.
The social media question is optional. The ESTA application will
clearly inform the applicant that answering it is optional.
How will CBP use my social media information collected through the
additional questions being added to the ESTA application in October
Information found in social media will enhance the vetting
process and may be used to review ESTA applications to validate
legitimate travel, adjudicate VWP ineligibility waivers, and identify
potential threats. If you choose to answer these questions and an
initial vetting by CBP indicates possible information of concern or a
need to further validate information, a highly trained CBP officer
will have timely visibility of the publicly available information on
those platforms, consistent with the privacy settings the applicant
has chosen to adopt for those platforms, along with other information
and tools CBP officers regularly use in the performance of their
For example, social media may be used to support or corroborate a
traveler’s application information, which will help facilitate
legitimate travel by providing an additional means to adjudicate
issues related to relevant questions about identity, occupation,
previous travel, and other factors. It may also be used to identify
potential deception or fraud. Social media may help distinguish
individuals of additional concern from those individuals whose
information substantiates their eligibility for travel.
DHS will handle social media identifiers in the same manner as other
information collected through ESTA. DHS has documented these
procedures in the updated ESTA System of Records Notice (SORN) and
Privacy Impact Assessment (PIA), which are available on the DHS
And to answer your questions...
What are the pros and cons of filling the "Online Presence" field?
Pros: Information found in social media will enhance the vetting process and may be used to review ESTA applications to validate legitimate travel, adjudicate VWP ineligibility waivers, and identify potential threats. Social media may help facilitate legitimate travel by providing an additional means to adjudicate issues related to relevant questions about identity, occupation, previous travel, and other factors.
Cons: none, except they can look into your social media.
Are there any situation where it is a good idea to fill it?
Yes, if you have nothing to hide and do not mind sharing your social media with CBP for investigation into your background. It can help verify your intention of travel and your legitimacy, completely at your discretion.