It depends who's doing the promotion and who they've worked with, their reputation is important. They also need to show you the whole promotion process and offer some kind of report or results. If someone can't reveal the people they've worked with then they might be a spammer.
You can even do it yourself
Use facebook post promo, it's cheap but only if you know what you're doing. However, there's a lot of good training online about that. You can advertise on any social platform to push your brand, all by yourself.
You can also buy review posts on popular blogs, buy a video review on a popular youtube channel, or even buy random weekly posts on a popular facebook page or twitter account.
If you choose to do it this way don't go to people who offer these services, most of them will give you fake views, likes, followers etc. You must find a regular person, build a good relationship with them and then pitch your offer. If you build a good relationship with them 1st, some might even promote you for free.
Hope that helps.