Greetings,
For a startup, it is undoubtedly a milestone that you have bagged a decent number of customers who are keeping your business on. Congratulations!
Further to answer your question about picking the first best fitted marketing channel for growth, I would strongly recommend ‘SEO’ as one of the best marketing channel, based on my research and experience. Let me give you come insights regarding my choice -
- SEO is easy to plan and gives you a much wider online presence as compared to other channels.
- Today, search engines are the largest platforms for any business to build their presence, reach their prospective clients, grow their audience and customer base.
- You can target and reach a large number of relevant audience for your business. SEO helps a business attract this audience and bring it to your website.

In simple words - How does SEO works?
Firstly, it helps a business identity the right audience or searchers who are relevant and interested in their products and services. Knowing this is the first step for any kind of marketing, and this way one better plan how and whom to target. This is done with the help of keyword research. Searchers use keywords to find information or products on search engines. When you target those keywords in your website then you can target those searchers. And they become your targeted audience. This helps you plan better for your online success with SEO.
Secondly, SEO optimizes your website for those keywords to help you in two ways. One, it helps in ranking of those keywords on top pages. Only the keywords that rank on the first page of search engines get the share of traffic from that keyword. Second, when you optimize your web page for a keyword, you optimize it for what your visitor is looking for. So you can serve those visitors better when they visit your page. This helps in better conversion and helps in generating leads from the traffic.
Third and important thing SEO does for your online success is make your website rank higher for your targeted keywords with various off page SEO activities.

Above should be enough for you to decipher if SEO is apt for growth of your business.

Further I would like to add, that as per Brian Balfour (renowned growth hacker), there are 5 scalable growth channels.
1) SEO: Mint, Hubspot
2) SEM / DISPLAY: TripAdvisor and other online comparison sites with high LTV eg. car insurance
3) FACEBOOOK: Zygna, Instagram, Upworthy
4) VIRAL / WORD OF MOUTH: Uber, Snapchat, Evernote, Groupon
5) SALES / PARTNERSHIPS: Salesforce, Oracle, IBM, PayPal
The most successful startups of the last decade, have scaled to millions of users globally by tapping into 1 or max 2 of these channels.

Hence you are right, it is not wise to start with both (GOOGLE/ FB). Above channels have played a key role in growth. However, you are the best judge for which one best fits your market and target audience.
In case, you need assistance in knowing the right channel for your business. Please feel free to share your business details with me to guide you through the best fitted channel of marketing for your business growth. I am just a phone call away.
https://clarity.fm/simkicity