Many clients and conversations we have with marketers ask us: Is social media worth any investment? The answer is yes, but maybe not from every channel that's available. Just make sure you set up measurement data points, implement tracking tags from Google Analytics (or some other program), and post relevant content to your audience(s) and not just "clutter."
You also have to look at what you deem as having value to your organization. If you see your audience, followers, comments, "retweets" are growing, and your Facebook, Twitter, Google+, Pinterest, YouTube/Vimeo, Linkedin and Instagram are all growing in numbers and in some cases driving people to your website, then it's valuable and driving awareness. More than likely, you will find a few of them are not worth any direct time investment.
True, social media doesn't allow you to measure it as closely as other Internet marketing pieces such as banner ads, Pay-Per-Click, or email marketing, but you can track direct leads and/or sales from each social media medium you use - if that is what you need to make sure the time, money, effort you spend on it is worth it. One key piece...don't be lazy. Don't just use a feeder system (like Tweetdeck or Hootsuite) that cuts off posts or uses hashtags where it's not appropriate. Dedicated users will know you did not intend that message for that medium and it can lose some impact.
You also have to find what works for your organization - every business is different based on size, geography, client demographics, etc. You need to watch the data and optimize for what works specifically for your business. We can tell you - just a bunch of self-promotion is NOT going to generate social media strategy that will maximize the efforts you are putting into it and there is no "cookie cutter" solution that will maximize results.
Social media also allows people to use the Internet on their terms. They may never leave Pinterest or Facebook until they are ready to transact. If you aren't there in that space, you could be missing 10-20% of your desired audience that didn't know about you because you weren't there and part of "their conversation." If you could get 10% more sales from having a solid strategy here, that could be a lot of growth - but it won't happen overnight.
The ultimate goal is to get a post in front of thousands through their networks - through the use of tagging, timely posts, and community/industry cross promotion. Marketing is the sum of all parts and the number of touch-points - something known as frequency - the number of times someone has to be exposed to something before they are willing to take an action. Through social media, you can increase those touch-points and be in front of them more often, which helps speed up the frequency time-period until an action will occur.
With an agency/resource who can monitor and report on social media for you (and also bring some creativity to your strategy), it can be the most effective $500-$1,500 a month (local-to-national type budgets) you spend out of your marketing budget and go a heck of a lot further than any advertising dollar could with reach. Just make sure you give it a few months and your resource for this is showing you the data and how it is moving in the right direction on the way there. Happy posting...