How to choose profitable bounty ? What factors we should analyse before pick any bounty 
I will try to help you:
1) the First is bounty Manager
2) Who in the table is in category the signature, if there is a hero or legendary, you should enter
3) try to find companies with less than 1000 members (TWITTER Facebook)
Perhaps these are the main criteria that I use.
Nowadays is it very difficult to find bounties with less participants than 1000. So I think that it is better to participate in both Twitter and Facebook bounties, to get from two different pools an allocation. Otherwise are the earnings less.
Only if you are talking about Social Networks and Media Campaigns, yes, there are a lot of people. But if you will seek good, you will find good projects with small amount of people. And dont try to focus at SM bountys, it's only for start, much better is signature campaign, video\article blog campaigns.