I’m wondering about necessity of posting regularly on my facebook page. Is it necessary to do that? or we just need a facebook page so we can advertising?
This actually depends on your goal.
If your goal is to drive traffic by letting your customers read a blog post, listen to a podcast episode, visit your landing page, or discover your app. Facebook will show your ad to those people within your target audience that are most likely to click on your link, based on their past behaviors, then you can use traffic objective on your FB ads.
However, when you want more people to engage with your ad. Engagements on your ads, include reactions, likes, comments, and shares. You can also use the Engagement objective when you wish to generate more Page Likes, Event Responses, or Offer Claims.
Choosing the Engagement objective can also assist you in building your Facebook Page Engagement Audiences that you can use for retargeting purposes for future advertising campaigns.