Giveaways can be expensive and attract users for the wrong reasons unless the thing being given away is incredibly aligned with what you offer. Many startups spend a lot of money trying to give things away only to find that a month later none of the users they attracted care about or use the product.

If you have a social network, your best means of attracting more users is to get the ones you already have to invite people. Although you can give them incentives for doing this, the best investment you can make is in making the product great, and specifically communicate how much better their experience will be if they invite their friends or colleagues. Figure out where in the user flow it makes the most sense to ask them to share the network or some content within the network with their friends. Perhaps right after they post, ask them if they want to share their post to Facebook or LinkedIn. Appeal to whatever emotion is driving their engagement, whether it is vanity, feeling good about doing something for a cause, or whatever, and encourage them to tell their own network in order to amplify what they will get out of it. If it makes sense, ask them to share their Google contact list and invite people en masse.

Once you figure out the smartest places to ask them to spread the word about your platform, get very focused on making it as easy as possible for them to do it. Measure the % of people who share the network and keep iterating how your sharing or contact invitation flow works. Push those metrics upward until you start seeing diminishing returns. Whatever is working, double down on it. Figure out what is causing it to work and see if you can duplicate it, or get more people to fall into a similar funnel.

Just keep in mind that nobody shares things that suck with their friends. First and foremost, make sure your product is loved by your users. If they love it, help them spread that love as easily as humanly possible.