With your first question/post, what maximum extent can you use Facebook in promoting your digital marketing company via Facebook in terms of a Free Advertising, this will depend on your the time you are willing to spend in your Social Media Strategy. As answered by our friends in this community, you need to build a network with your related niche (a place where your expertise is suited), as well as those who will be interested in the product/service you will be providing.
Of course, in building a community, you have to build trust in your company/brand and this is what is going to eat most of your time. You have to get people's attention, remain professional, and at the same time be informative that you will be one of the priority pages viewers will be visiting whenever they have a query for certain topics which you basically offer.
Building a community is such a broad topic which generally includes gaining likes on your fan page, comments and views on the contents/images/videos/podcasts you share, and other things you are sharing and posting on your page.
And as for your follow up question with @Webexperts, do you need to add requests to people... This again will be based on your "Building a Community" strategy. If I were you, I would. Most especially if those people you will be sending your requests can really help you in the long run. This is just my point of view, but I believe if you have something worthy to share, it won't hurt to request from them (and if you won't be flooding their news feed anyway).