Having a strong online presence surely helps a lot, but not a must.
As an applicant to a job, you need to show them a bit of what you are capable of to get an interview generally, assuming the firm has a large pool of applicants to choose from.
The review process really changes a lot depending on type of the firm, location and the open position. Some firms doesn't even bother to search online, and some will only do a small search. But, especially for popular firms and for good paying positions, here is what goes on generally:
- They will post some ads for the open position, requiring you to fill out a form or asking for resume or requiring a solution to a problem they posted.
- They will sift through the applications, eliminating some percent of them by looking at the answers or resume.
- They will search the applicants that passed the first stage online, on Google and other social sites. They will check blogs, github, bitbucket, SE like sites, mailing lists, forums etc. to get to know you a little bit more, to see if you worth an interview. They will eliminate some of the applicant at this phase also.
- At this stage, generally they will only have a small amount of applicants left, and they will start interviews.
Now, if you have a strong resume (work experience in related areas, good references etc.), the online search part may not matter much.
But, if you don't have much to write at your resume, your online profile becomes the main resource for the interviewer to learn about you. If you don't have much to show at that stage, and there are other applicants that have a resume like you but have a strong online presence to show them they are capable of doing good work, they will get bonus points.
The main point is, you need to get ahead of other applicants to get an interview, and having a strong online presence helps with that. It's not a must, but generally its a plus.