For some, growing a Social Network is more about numbers, more than a two way communication. The goal, "document" popularity by having many followers, is a strategy many have, so buying followers or follow, hoping that the user will follow back, is common.
Even this strategy may have a value, besides the illusion of being popular. A small percentage will always see a tweet or a Facebook post, but being realistic, quantity instead of quality have one purpose only.
So, what's the best way to do it, if you want to become "popular"?
There are many tools if you want quantity, many followers. Tools where you, in a way, buy followers, or get paid by others to follow them, paid with credits (points) you may use to buy your followers by offering the same back to others.
So, if you still want to use the quantity strategy; where can you find a site that let you buy followers, or get paid to follow others?
Twiends.com
The best known may be Twiends.com - It used to be possible to get both Twitter and Facebook friends, but now it is Twitter only.
Offering 2 seeds (what they call a credit on Twiends), is usually enough to grow your network. Offering more than 2 will give you followers faster, but of course, not that many. 2 seems to be OK if you have a little patience.
Buying credits : 1250 seeds / US$ 29,95
Followers/US$: 21 (offering 2 seeds)
YouLikeThis.com
This site seems to have more users than Twiends. Maybe because you can get Facebook "Likes" also. Not only Twitter.
If you also want traffic to your YouTube videos and web-site, which may be a better idea than just collecting followers, you can get it from YouLikeThis.
Offering 2 points seems to give a new follower every 30 minutes, so it's a question of time or as many as possible at a lowest possible cost.
You can claim 50 credits daily, if you log in an claim the credits.
Buying credits : 1250 points / US$ 17,50
Followers/US$: 36 (offering 2 points) - (24 - if offering 3 points)
Friendrise
This site seem to work better for Facebook than Twitter, but I'm not sure it works better for Facebook than YouLikeThis.
The system is a little different when it comes to credits. You get more credits, for following, but you have to offer more to get followers.
A daily bonus of 50 credits if you log in and claim the credits.
Buying credits : 1000 credits / US$ 17,50
Followers/US$: 6 (offering 10 credits)
It might be that the cost using Friendrise is due to a low number of users, so a long term strategy offering 2 credits may be better and of course more value for money.
Conclusion:
Which one is the best? It's between Twiends and YouLikeThis, but only if you want only Twitter "friends". Twiends will give you friends faster, but at a higher cost. If time isn't in focus, YouLikeThis is a much better option since it will give 50% more.
If focused on Facebook, then, without a doubt, YouLikeThis is what you should choose.