When starting a new venture you will try reducing your risks to the minimum, this means spending little and getting a lot from your investments. These are some of the best tactics you can use to save your pennies and jumpstart your initial social audience. 

Common to all networks

These can be used in any social network (Facebook, LinkedIn, Google+, Youtube, etc..).

1- Invite your friends, get your team to invite theirs to like/follow your page

This will give you an initial audience of promoters that know about your new venture and want to support it.

2- Add respective like/follow/retweet buttons to your webpage/app

3- Include links for your main social pages in your email/newsletter signature and website

4- Member get Member (or Referrals)

Give freebies/discounts to people that refer friends that then go and purchase from you. If done on a cost per sale basis you can acquire customers whilst remaining ROI efficient.

X (Twitter)

5- Engage high social value people that ask questions to their audience

6- Favourite any interesting tweets you find in your industry

Favourites have a high correlation with people following you back. A good way of doing it is just searching for an interesting #hashtag and trailing through the search results.

Note: There are some tools that do this automatically (FollowGen and TargetPattern ), but these come with certain risks (imagine favouriting automatic a comment that completely goes against your companies’ values?)

Facebook

7- Interact with other fan pages

Although you can't/shouldn't just post things in other pages you can interact with insightful comments/likes on other page's posts. This will give you brand more exposure especially if you target very popular posts, where several people will receive a newsfeed update with your comment.

9- Do joint ventures

Message interesting pages that you think your target audience follows and propose doing a featured post about your website. You can then retributed the favour on your own page. 

10- Promote your account

Just click the big blue button saying promote in your Facebook page. Initial costs can be as low as £3/day to obtain from 1-5 followers a day.

Sample ad 

11- Promote contests

"Like a post", "share a photo with us" or connect to our app Contests are a great way of getting email addresses, generating word of mouth and getting more followers/likes , in fact they were so popular and "abused" that Facebook changed the rules in the end of August'13 so before doing it catchup at https://www.facebook.com/page_guidelines.php.

LinkedIn

12- Share your blog posts in interesting groups for your audience. 
Invite members of your team of doing it as well. Showing you have a top performing team writing, commenting on real issues proves that you are a industry leader.

13- Follow people that like your posts see how they behave and connect to them when the time is right. 

14- Contact interesting users using the internal messaging system

Make this as customised as possible, it is easy to spot SPAM messages from authentic and honest propositions. If this proves to work get a premium account and increase reach and number of messages you can send.

15- Promote the story of your company or a group post that was particular successful

Google+

16- Connect your website to your publisher Google+ profile, identify your authors with meta tags

17- Enable paid search social extensions

18- Engage people with an interesting profile and invite them to post in your blog

This will prove to be useful both in terms of attracting their followers to your website, as well as increasing your website ranking.

19- Create a community that represents your industry

Luckily Google+ is still so new that there is still space to create unique and new communities around your industry and own them. If you build a community avoid spamming it with sales messages, you will be able to crowdsource your content from its users and leverage on this to deliver extra value to your followers later. 

Integrate

20- Integrate your stories, social media presence and tactics cross different social platforms. This way you can re-engage your users in different media places (i.e. story/article/contest that starts on Facebook, but asks the user to tweet something/followed by sharing a photo on Google+).