There are many social media campaign around. If you like, you can join them and earn money from tweet and retweet. Try to apply in one campaign to test and if you have experience already, you can apply upto 10 campaign at a time 
Yes, you're true 
)) with twitter or any other social media you can join how many campaigns you want, it's different with signature campaign, you can only join 1 at a time but when you join too much campaign you will have a lot of mistake such as forget retweet or tweet for any campaign )) so I think you should join more than one twitter campaign but not too much