Which one of these should startups focus on for the most impact:
The biggest downside of focusing on SEO first is that almost every month Google makes a white-hat tactic negative (for example: guest posts are apparently bad now, which were always a white-hat way to get exposure).
They are all related.
Content marketing is considered one of SEO tactics, it's most time consuming but very effective, if done right, and popular. Social media marketing can be a part of your SEO and/or branding strategy, unless it's an area you can actually use for generating leads (not all companies use it that way). A lot of companies don't get a positive ROI on social media, but they spend effort on it anyways because social signals are a factor in search engine rankings (aka SERPs).
It's a misconception that SEO is so fragile - it is only so if you don't balance out your strategy and chase quick wins by overusing any one tactic in a short period of time. If you do it yourself, don't outsource and apply common sense, you are unlikely to get penalized.
SEO takes time, so you don't want to postpone it for too long. Start scratching and learning bit by bit, your skills and effectiveness will improve overtime.
The key to successful SEO is BALANCE. Here is a tip list:
1. Don't throw a lot of effort on it just once in a while - you want links to your site be added naturally and on ongoing basis without any spikes.
2. Balance links between nofollow (blog comments) and follow.
3. Don't go after links from sites not relevant to your niche, no matter how easy they are to get. Context is most important factor for links. Panda penalized sites with a lot of links from unrelated sites.
I encourage SEO because it has given me the best ROI for my business (I do have a lot of content though), nothing else comes even close. It's also fairly easy to measure (over time), so can be very encouraging.