Heya!
Advertisement is indeed very important for any game to make it a success. There are so many cool games available on the market that it is of big importance that your game stands out. You can consider hiring a PR agent or agency, but it all depends on your budget.
A PR agent or agency will most likely ask you: 'what is your budget' and 'what do you have in place'? If you don't have a large budget the budget for social media campaigns will be skimmed down or they won't personally contact journalists but send out general e-mails to websites like gamespress.com. Budget, budget, budget is the key of what they are able to offer. If they need to build up a network from scratch (0 followers, 0 likes etc) they will ask for a lot more money, because they need to grow these channels. If you already have established a community with hundreds/thousands of followers/likes with a solid plan in place they will charge you less, since they only need to maintain it.
My advice to you is:
1) Register all domains you need;
2) Register all social media channels you want to use (a Twitter username, Facebook page, YouTube, Twitch etc)
3) Create a press kit (
http://dopresskit.com/)
4) Make a social media calendar (when you want to release content and on what channel for the upcoming months)
5) Write a press release (draft) of the information that you want to share with the public. Keep in mind that '6 playable maps' is not really a USP (unique selling point) but features like: '4 player co-op mode' are selling points.
6) Make a list of journalists / websites that you want to contact and match your target audience. If you publish a co-op game it is highly recommended that you get an article published on co-optimus.com
7) Release and produce content to share and start building a network
Try to do as much work yourself, or find a hobbyist PR-guy that can help you for free with the promise of compensation when revenue is being made. A
great PR-agency can be pretty expensive ($500 just for an introduction conversation is what I have experienced once)
I hope this helps!